System and method for storing ammonia

Abstract

A system (200, 200') for safely storing ammonia below ground level, comprising: a storage tank (300, 300') located underground for the storage of liquid ammonia; and a safety system (400, 400') connected to the storage tank (300, 300') to control leaks from the storage tank (300, 300') and allow transfer of liquid ammonia into and out of the storage tank (300, 300'), the safety system (400, 400') comprising: a reliquification system (410, 410') configured to receive and liquefy gaseous ammonia boiled from the liquid ammonia in the storage tank (300, 300') and deliver the liquified ammonia to the storage tank (300, 300'); a collection tank (420, 420') configured to collect leaked liquid ammonia from the safety system (400, 400') and deliver the leaked liquid ammonia to the storage tank (300, 300'); a scrubber (430, 430') configured to treat gaseous ammonia leaked from the safety system (400, 400') and deliver treated gas to the atmosphere or the sea through a carbon filter; a water tank (440, 440') connected to the scrubber (430, 430') and configured to receive contaminated water; a backup external cooling system (450, 450') configured to be operable to provide backup cooling to the storage tank (300, 300') and collection tank (420, 420'); and a liquid ammonia transfer system (460, 460') configured to transfer liquid ammonia into and out of the storage tank (300, 300'); wherein the reliquification system (410, 410') is configured to provide cooling to the storage tank (300, 300') and the collection tank (420, 420') as a result of the liquification of ammonia gas.

Description

[0001] SYSTEM AND METHOD FOR STORING AMMONIA

[0002] FIELD

[0003] The present invention relates to the storage of ammonia. More particularly, the present invention relates to the safe storage of ammonia close to human populations, such as in or around city ports.

[0004] BACKGROUND

[0005] Hazardous fluids are widely used and stored in industry for myriad purposes. Some hazardous fluids should not be leaked, but if a leak does occur, they can be relatively easily contained without immediate or severe impact. On the other hand, some hazardous fluids can cause immediate and / or severe impact to people and / or the environment if they are leaked.

[0006] Ammonia is a widely used chemical with many applications. Ammonia is an inorganic compound of nitrogen and hydrogen with the formula NH3. Ammonia is commonly stored in refrigerated ammonia tanks. In liquid form, ammonia is stored at around -33 degrees Celsius at atmospheric pressure. Ammonia can also be stored at ambient temperature under high pressures. Additionally, ammonia can be stored in a semi-refrigerated state, at an intermediate pressure and temperature.

[0007] Examples of ammonia applications are as fertilizer in agriculture, as energy carrier for storage of excess energy from energy plants, and as fuel in the shipping industry and power generation.

[0008] For the latter two examples, there is typically a need for storing large amounts of ammonia. When used to fuel ships, the storage tanks should preferably be close to harbours where bunkering operations take place.

[0009] However, ammonia is toxic, and safety measures must be taken to avoid the risk of exposure to people. It is critically important that ammonia does not leak, in gas or liquid form, during storage of ammonia. Leaked ammonia liquid may reach the atmosphere and boil. If leaked ammonia is leaked into the sea it may reach the sea water surface and boil uncontrollably. Ammonia is very damaging for marine life and fish. It is therefore critically important that ammonia does not leak from a storage tank.

[0010] As ammonia emerges as a valuable fuel source for shipping, it will often be required that the vessels can dock and refuel in city ports, or other ports close to larger populations or infrastructure.

[0011] It is highly desirable to provide an ammonia storage system and method for storing ammonia which is safe such that the system can be located very close to population centres.

[0012] Currently, ammonia is stored in large tanks at or close to ports. Ammonia fuelled ships must be brought close to the tanks for bunkering of ammonia, i.e. the transfer of ammonia from the storage tank to the ship.

[0013] The large storage tanks consume much valuable space, particularly in busy city ports. Furthermore, the large storage tanks are undesirable for many of the population surrounding the port as they are so large and obstruct much of the skyline. Residents in the population may also feel unsafe living very close to visible large chemical storage tanks. Finally, there are often safety zones imposed around large ammonia storage tanks. Within the safety zones, people cannot work or live, therefore the effective space consumed by ammonia storage tanks is often much larger than their actual physical space consumption.

[0014] It is highly desirable to provide systems and methods for storing ammonia in populated areas such as city ports which addresses at least some of the aforementioned problems.

[0015] SUMMARY

[0016] According to a first aspect of the invention, there is provided a system for safely storing ammonia below ground level, comprising: a storage tank located underground for the storage of liquid ammonia; and a safety system connected to the storage tank to control leaks from the storage tank and allow transfer of liquid ammonia into and out of the storage tank, the safety system comprising: a reliquification system configured to receive and liquefy gaseous ammonia boiled from the liquid ammonia in the storage tank and deliver the liquified ammonia to the storage tank; a collection tank configured to collect leaked liquid ammonia from the safety system and deliver the leaked liquid ammonia to the storage tank; a scrubber configured to treat gaseous ammonia leaked from the safety system and deliver treated gas to the atmosphere or the sea through a carbon filter; a water tank connected to the scrubber and configured to receive contaminated water; a backup external cooling system configured to be operable to provide backup cooling to the storage tank and collection tank; and a liquid ammonia transfer system configured to transfer liquid ammonia into and out of the storage tank; wherein the reliquification system is configured to provide cooling to the storage tank and the collection tank as a result of the liquification of ammonia gas.

[0017] The storage tank may be configured to withstand up to at least 2 bar internal pressure or at least 3 bar internal pressure or at least 4 bar internal pressure or at least 5 bar internal pressure.

[0018] The reliquification and collection tank may be configured to withstand an internal internal pressure.

[0019] The storage tank may have a storage capacity of between 1 ,000m3and 40,000m3.

[0020] The storage tank may comprise walls, a floor and a roof, wherein the walls and / or floor are made of concrete and the roof comprises a steel structure.

[0021] The storage tank may comprise walls, a floor and a roof, wherein the walls and / or floor are made of concrete and polymer and the roof comprises a steel structure.

[0022] The concrete and polymer may be formed in alternating layers.

[0023] The roof of the storage tank may be covered with earth.

[0024] The roof may be welded or bolted to the walls and / or floor.

[0025] The system may further comprise: a first fluid communication conduit fluidly connecting the reliquification system and the collection tank; and a second fluid communication conduit fluidly connecting the collection tank and the storage tank; wherein the reliquification system is configured to provide cooling to the first and second fluid communication conduits as a result of the liquification of ammonia gas.

[0026] The reliquification system may be configured to provide cooling of the first and second fluid communication conduits such that the first and second fluid communication conduit internal temperatures are between -10 and -35°C or around -20°C.

[0027] The water tank may have a capacity of between 100m3and 15,000m3.

[0028] The system may further comprise a chimney connected to the scrubber to allow venting of treated gas to the atmosphere.

[0029] The reliquification system may be configured to provide cooling of the storage tank such that the storage tank internal temperature is between -5 and - 50°C or between -30 and -40°C.

[0030] The reliquification system may be configured to provide cooling of the collection tank such that the collection tank internal temperature is between -10 and -35°C or around -20°C.

[0031] The backup external cooling system may comprise at least one freeze pipe located in the ground sufficiently close to the storage tank such that in use the at least one freeze pipe can cool the storage tank such that the storage tank internal temperature is between -5 and -40°C or between -30 and -40°C.

[0032] The at least one freeze pipe may be located in the ground within 2 to 10m or 4 to 8m or around 5m from the storage tank.

[0033] The at least one freeze pipe may be located in the ground sufficiently close to the collection tank such that in use the at least one freeze pipe can cool the collection tank such that the collection tank internal temperature is between - 10 and -35°C or around -20°C.

[0034] The at least one freeze pipe may be located in the ground within 2 to 10m or 4 to 8m or around 5m from the collection tank.

[0035] The at least one freeze pipe may comprise a plurality of freeze pipes.

[0036] The backup external cooling system may further comprise a removable cooling unit configured to connect to the at least one freeze pipe to provide cooling of the at least one freeze pipe.

[0037] According to a second aspect of the invention, there is provided a method of safely storing ammonia underground, comprising the steps of: providing a system according to the first aspect of the invention; providing a volume of liquid ammonia in the storage tank; operating the reliquification system to: receive and liquefy gaseous ammonia boiled from the liquid ammonia in the storage tank and deliver the liquified ammonia to the storage tank; and provide cooling to the storage tank and the collection tank as a result of the liquification of ammonia gas.

[0038] The method may further comprise the steps of: collecting leaked liquid ammonia from the safety system in the collection tank and delivering the leaked liquid ammonia to the storage tank.

[0039] The method may further comprise the steps of: treating gaseous ammonia leaked from the safety system in a scrubber and delivering treated gas to the atmosphere or the sea through a carbon filter; and collecting contaminated water from the scrubber in the water tank.

[0040] The method may further comprise the step of: operating the backup external cooling system to provide cooling to the storage tank and collection tank. The method may further comprise the step of transferring liquid ammonia into the storage tank using the liquid ammonia transfer system.

[0041] The method may further comprise the step of transferring liquid ammonia out of the storage tank using the liquid ammonia transfer system.

[0042] BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Embodiments of the invention will now be described with reference to the following drawings, in which:

[0044] Figure 1 shows a first view of an ammonia storage system located underground in a city port;

[0045] Figure 2 shows an alternative view of the ammonia storage system of Figure 1 ;

[0046] Figure 3 shows a cross-sectional view through a first example of an ammonia storage system; and

[0047] Figure 4 shows a cross-sectional view through a second example of an ammonia storage system.

[0048] DETAILED DESCRIPTION OF THE DRAWINGS

[0049] Figures 1 and 2 shows a site 100 where it is desirable to store large quantities of liquid ammonia. The site 100 comprises a harbour 101 , a plurality of industrial buildings 102, agricultural land 103 and domestic dwellings 104. It will be understood that in other sites where it is desirable to store large quantities of liquid ammonia, such as large city ports, there may be many more buildings, infrastructure, dwelling and other aspects of the built environment nearby. At the harbour 101 there is a ship 105 docked and connected to an ammonia storage system 200 for storing liquid ammonia. It will be understood that the ship 105 may be receiving or delivering liquid ammonia to the ammonia storage system 200.

[0050] In Figures 1 and 2 it can be seen that in the presently described example the ammonia storage system 200 is located just below ground level within the harbour 101 area. It will be understood that in alternative examples (not shown) the ammonia storage system 200 may be located below sand, rocks, earth, mud, clay or any other material found at the surface of the earth. Furthermore, in alternative examples, the ammonia storage system 200 may be located within a mountain, such as in a cavern blown into or excavated from a mountain. The term mountain is used herein, and it will be understood that the term is used broadly and may refer to rolling hills for example. Further details of the construction of the ammonia storage system 200 are provided later after description of the components of the system 200.

[0051] Referring to Figure 3, details of the major components of the system 200 are now provided. The system 200 comprises a storage tank 300 comprising side walls 301 , a floor 302 and a roof 303. The side walls 301 and floor 302 in the presently described example are formed of concrete and polymer. In other examples, the side walls 301 may be constructed as secant walls which provides a suitable construction method for forming a sealed storage tank 300 within a cavern in the ground. Alternatively, the side walls 301 may be formed of steel. Alternatively, the side walls 301 may be formed as slurry walls. Alternatively, the side walls 301 may be formed of one or more flexible membranes.

[0052] In some examples the side walls 301 will be formed of only concrete. In some examples the side walls 301 will be formed of several alternating layers of concrete and polymer. Polymer may be used when the side walls 301 are being constructed in unstable rock and / or when an extra safety barrier is required.

[0053] The roof 303 in the presently described example is formed of steel in a truss structure. In some examples the roof 303 may be welded to the side walls 301 . In some examples the roof 303 may be bolted to the side walls 301 . The roof 303 may be constructed to withstand high internal pressure by formation of the roof 303 in a curved or double-curved shape in some examples. Additionally, or alternatively, the roof 303 may be provided with vertical tension rods that are connected to the side walls 301 or floor 302. The storage tank 300 in the presently described example has a capacity of around 15,000m3. In some examples the storage capacity may be between 1 ,000m3and 40,000m3.

[0054] As can be seen in Figure 3, the storage tank 300 is located below ground level. In the presently described example, the roof 303 is only marginally below ground level, and is covered by a thin layer of rocks and earth 304. It will be understood that in alternative examples the storage tank 300 may be located significantly deeper underground. That is to say, in some examples there may be a thicker layer of rock and earth 304 covering the roof 303. In some examples the layer of rock and earth 304 may between 3m and 20m thick. Still referring to Figure 3 it can be seen that the storage tank 300 is connected to a safety system 400 to control leaks from the storage tank 300 and allow transfer of liquid ammonia into and out of the storage tank 300. The safety system 400 comprises a reliquification system 410, a collection tank 420, a scrubber 430, a water tank 440, a backup external cooling system 450 and a liquid ammonia transfer system 460. Each of these components will be explained in more detail in due course.

[0055] Liquid ammonia at atmospheric pressure has a boiling point of around - 35.7°C. Therefore, the storage tank 300 comprising liquid ammonia must be kept at a very low temperature at all times to ensure rapid and uncontrollable boiling of the ammonia does not occur.

[0056] It is highly desirable to keep the liquid ammonia at a temperature below - 37.5°C to ensure no boiling occurs, however this is challenging and realistically technically difficult, particularly when the storage tank 300 is used for loading and unloading of ammonia such as bunkering of ammonia to ships for example.

[0057] Therefore, it is accepted that some (albeit very little) boiling of ammonia occurs, thereby producing gaseous ammonia.

[0058] The storage tank 300 in the presently described example is configured to withstand up to at least 2 bar internal pressure. In some examples, the storage tank 300 may be configured to withstand up to at least 3 bar internal pressure or at least 4 bar internal pressure or at least 5 bar internal pressure. The storage tank 300 is configured to withstand an internal pressure such that the internal pressure can safely rise within the storage tank 300 without causing explosion or rupture of the storage tank 300. Although it is highly desirable to minimise ammonia boiling, some boiling is inevitable. Boiling of ammonia inside the storage tank 300 causes an increase in internal pressure which the storage tank 300 must be able to withstand.

[0059] Furthermore, it is highly desirable that the storage tank 300 is configured to withstand a much greater internal pressure than the expected internal pressure in operation, i.e. the internal pressure produced when a small amount of ammonia boils due to heat entering the storage tank 300 during transfer of ammonia into or out of the storage tank 300 in use. It should be noted that the internal temperature in the storage tank 300 may also rise for other reasons. This provides a safety factor such that in use if maintaining the storage tank 300 at a sufficiently low temperature to minimise boiling of ammonia is not possible then the internal pressure within the storage tank 300 can safely rise for a reasonable length of time until backup cooling can be provided and / or the storage tank 300 can be emptied of ammonia. This will be explained in more detail later.

[0060] The reliquification system 410 is configured to receive and liquefy gaseous ammonia boiled from the liquid ammonia in the storage tank 300 and deliver the liquified ammonia to the storage tank 300. Some boiling of ammonia in the storage tank 300 thereby becomes acceptable since the gaseous ammonia is converted back to a liquid and is then delivered back to the storage tank 300. A convenient advantage of the reliquification system 410 is that the process of converting gaseous ammonia back into liquid ammonia produces a cooling effect. That is to say, the reliquification system 410 serves to provide cooling when operating. Said cooling cools the storage tank 300 to attempt to keep the internal temperature of the storage tank 300 as low as possible.

[0061] Said another way, the operation of the reliquification system 410 may be viewed as a cyclic process with the storage tank 300. At the start of the cycle, some of the liquid ammonia in the storage tank 300 boils because the internal temperature of the storage tank 300 is greater than the boiling temperature of the ammonia at the pressure inside the storage tank 300. The gaseous ammonia is fed to the reliquification system 410 where it is liquified and during liquification the reliquification system 410 provides cooling to the storage tank 300, thereby supressing further or uncontrolled boiling. In this regard, the reliquification system 410 serves to provide cooling to the storage tank 300 when cooling is required because the liquid ammonia inside the storage tank 300 is boiling due to the temperature of the liquid ammonia. The reliquification system 410 in the presently described example is configured to provide cooling of the storage tank 300 such that the storage tank 300 internal temperature is between -5 and -50°C. In some examples the reliquification system 410 may be configured to provide cooling to the storage tank 300 such that the storage tank 300 internal temperature is between -30 and -40°C.

[0062] Viewed another way, for explanation purposes only, the reliquification system 410 may be seen as a refrigeration system for the storage tank 300, whereby the refrigeration is only provided when the temperature inside the storage tank 300 becomes high enough that boiling of the liquid ammonia occurs. Rel iquification systems are known in the art and it will be within the capabilities of a person skilled in the art to select a suitable reliqu ification system for the described purpose.

[0063] It will be appreciated that some liquid ammonia may leak from the safety system 400 during operation. For example, some liquid ammonia may leak from the reliquification system 410. The collection tank 420 is configured to collect leaked liquid ammonia from the safety system 400 and deliver the leaked liquid ammonia to the storage tank 300.

[0064] Although not shown in Figure 3, the safety system 400 comprises a first fluid communication conduit fluidly connecting the reliquification system 410 and the collection tank 420 and a second fluid communication conduit fluidly connecting the collection tank 420 and the storage tank 300. In some examples, the reliquification system 410 may deliver the liquified ammonia to the storage tank 300 via the collection tank 420. In alternative examples the reliquification system 410 may deliver the liquid ammonia directly to the storage tank 300. In either case, the first fluid communication conduit may be configured to catch any leaked liquid ammonia from the reliquification system 410 and deliver it to the collection tank 420. In the presently described example, the reliquification system 410 and collection tank 420 are configured to withstand an internal pressure.

[0065] It will be understood that the collection tank 420 is intended to collect liquid ammonia and maintain the liquid ammonia in the liquid phase within the collection tank 420 before it is delivered to the storage tank 300. It is further intended that the first and second fluid communication conduits also maintain the liquid ammonia in the liquid phase during transfer of the liquid ammonia therethrough. In this connection, the collection tank 420, first fluid communication conduit and second fluid communication conduits are all maintained at a low temperature to maintain liquid ammonia therein in the liquid phase. Cooling of the collection tank 420, first fluid communication conduit and second fluid communication conduit may be provided by the reliquification system 410 in the same way as the reliquification system 410 provides cooling to the storage tank 300 (as described above). In addition to this cooling, or alternatively, the collection tank 420, first fluid communication conduit and second fluid communication conduit may be kept cool by an external cooling system. It will be appreciated that the reliquification system 410 and fluid communication conduits may be significantly smaller than the storage tank 300, therefore it may be possible to provide cooling of these components by an external cooling means. However, it is particularly convenient if the cooling of these components is provided by the reliquification system 410, thereby not requiring external cooling systems.

[0066] Regardless of the means for cooling for collection tank 420 and first and second fluid communication conduits, it is preferable to maintain the internal temperature of these components to between -10 and -35°C or around -20°C.

[0067] Still referring to Figure 3, the scrubber 430 is configured to treat gaseous ammonia leaked from the safety system 400. Scrubbers 430 are known in the art and it will be within the capabilities of a person skilled in the art to select a suitable scrubber 430 for the purpose of the treatment of gaseous ammonia. In some examples, the scrubber 430 may be connected to a chimney 431 arranged to deliver treated gaseous ammonia to the atmosphere. After treatment, the gaseous ammonia is actually mostly air with minimal ammonia in the air. Therefore, the treated gas can be simply expelled to the atmosphere without posing any danger to humans or producing odour on land. For a 15,000m3storage tank, as in the presently described example, the chimney 431 may be arranged to expel the treated gas at around 15-20m above ground level.

[0068] In other examples, the treated gaseous ammonia may be delivered into seawater by connection of the scrubber 430 directly to an outlet pipe located under the sea. In some examples, the safety system 400 may comprise both a chimney 431 for venting treated gaseous ammonia to the atmosphere and an outlet pipe located under the sea for delivery of treated gaseous ammonia to the sea water. In such cases it may be possible to switch between venting to the atmosphere and discharging the treated gas to the seawater. In some cases the ability to switch between discharge to the atmosphere, i.e. the air, and discharge to seawater may only be provided as redundancy to ensure that treated gaseous ammonia can always be discharged if it becomes dangerous or undesirable to continue releasing treated gaseous ammonia in the preferred way. For example, if the system 400 is configured to release treated gaseous ammonia to the atmosphere via a chimney 431 as previously described and during operation a person must work in the vicinity of the chimney 431 , it may be highly desirable to switch to discharge to seawater, if the system is configured to provide such ability to switch. Further, if the preferred discharge method, such as for example discharge to the atmosphere via the chimney 431 , is not possible because of mechanical blockage of the chimney 431 , the system 200 may instead discharge to the seawater, thereby still allowing safe discharge of gaseous ammonia from the system 400, thereby providing redundancy and improving the safety of the system 200. In some examples, discharged ammonia to the atmosphere may be flared.

[0069] Where the gaseous ammonia is discharged into seawater, the scrubber 430 is preferably arranged with an activated carbon filter (not shown) such that the gaseous ammonia is delivered to the seawater through the activated carbon filter. Furthermore, it is preferable that the gaseous ammonia is released into the seawater at a suitable depth. As an example only, the gaseous ammonia may be released into the water at a depth of around 10m. Furthermore, at the discharge location in the sea a device may be provided which is configured to shatter the gaseous ammonia bubbles, thereby preventing the gaseous ammonia bubbles from quickly reaching the water surface and ammonia being absorbed into the water. An example of one such suitable device may be a multi-story tank of perforated plates. It will be understood that other suitable devices may also be used.

[0070] The water tank 440 is fluidly connected to the scrubber 430 such that contaminated water from the scrubber 430 can be collected in the water tank 440 in use. The water tank 440 may be configured to hold contaminated water for a period of time until the water tank 440 can be emptied. In this connection, the water tank 440 may have a capacity of between 100m3and 15,000m3such that a sufficient quantity of contaminated water may be stored therein.

[0071] The water tank 440 may be retrievable from the system for emptying or may comprise a fluid port or connection to above ground level for discharge of contaminated water to a suitable collection vessel above ground.

[0072] In some examples, if the water tank 440 is provided with a sufficiently large capacity to hold a sufficiently large quantity of water in the water tank 440, the water tank 440 may be further configured to receive liquid ammonia, such as liquid ammonia leaked from other components of the safety system 400 for example. In this connection, the safety system 400 may be arranged such that leaked liquid ammonia is delivered to the water tank 440 where it can mix with the water in the water tank 440 and thereby safely be absorbed into the water. It should therefore be noted that the water tank 440 must be sufficiently large in such cases where it is used to collect leaked ammonia, such that it can be safely absorbed in the water.

[0073] The backup external cooling system 450 is configured to be operable to provide backup cooling to the storage tank 300 and collection tank 420. In this connection the backup external cooling system 450 in the presently described example comprises a plurality of freeze pipes 451 and an external cooling unit 452. The plurality of freeze pipes 451 are inserted into the earth such that they surround the storage tank 300. In this connection, the freeze pipes 451 are located sufficiently close to the storage tank 300 or directly adjacent the storage tank 300, such that a sufficient cooling effect can be provided to the storage tank 300. In some examples the freeze pipes 451 may be located in the ground within 2m to 10m from the storage tank 300. In some examples the freeze pipes 451 may be located in the ground within 4m to 8m from the storage tank 300. In some examples the freeze pipes 451 may be located in the ground around 5m from the storage tank 300. In some examples, a single freeze pipe 451 may be provided. In such cases it may be preferable that the freeze pipe 451 is provided as a continuous loop around the storage tank 300. Where a plurality of freeze pipes 451 are provided, it is preferable that the plurality of freeze pipes 451 are evenly distributed around the storage tank 300. In the described example, the freeze pipes 451 run down the side walls 301 of the storage tank 300. It will be understood that in other examples not shown the freeze pipes 451 may also run below the floor 302 of the storage tank 300.

[0074] In use, the external cooling unit 452 can be connected to the freeze pipes 451 to drastically reduce the temperature of the freeze pipes 451 such that the freeze pipes 451 can provide cooling to the storage tank 300. Although the freeze pipes 451 are installed within the ground adjacent the storage tank 300, it is possible but not necessary that a permanent external cooling unit 452 is provided at the surface. Instead, the freeze pipes 451 may comprise a connection such that the freeze pipes 451 can be connected to the external cooling unit 452 when required. In this way, the external cooling unit 452 may be connected to the freeze pipes 451 only when required, and may be utilised at another location when not required. This saves space on the land above the storage tank 300 and also reduces the permanent equipment needed at the location. Furthermore, a sufficiently powerful external cooling unit 452 may be expensive, therefore it may provide economic advantages to only provide an external cooling unit 452 when it is actually required when backup cooling is needed.

[0075] The liquid ammonia transfer system 460 is configured to transfer liquid ammonia into and out of the storage tank 300. In the presently described example, the liquid ammonia transfer system 460 comprises a safety hose 461 configured such that leaked liquid ammonia being transferred by the safety hose 461 in use will be caught by the safety hose 461 and can be transferred back to the storage tank 300 or the collection tank 420.

[0076] Still referring to Figure 3, details of one possible construction method to form the storage tank 300 within the ground is now described. As previously discussed, the storage tank 300 may be located in a cavern blown into or excavated from a mountain. The preferred method of forming the cavern is by means of mechanical excavation. In some cases, the storage tank 300 is preformed and delivered to the cavern as a substantially complete tank 300. In other cases the storage tank 300 is constructed within the cavern, as is now explained. However, construction of the storage tank 300 within the cavern is problematic due to the fact that the cavern may fill with water as the excavation will be deeper than the water table.

[0077] The preferred solution to this is to begin construction of the storage tank

[0078] 300 with installation of the freeze pipes 451 surrounding where the storage tank

[0079] 300 shall be located. In this connection, the freeze pipes 451 are located in position in drilled or excavated holes in the ground. The freeze pipes 451 are then connected to a cooling unit 452 and operated to provide freezing of the ground which freezes the water table in the location where the excavation will be conducted. During excavation, the water table remains frozen and the cavern can be excavated without filling with water. After the cavern has been formed and is free of water, concrete and polymer can be sprayed or otherwise attached to the inside walls of the cavern to form the side walls 301 and floor 302 of the storage tank 300. In some examples alternating layers of concrete and polymer may be used. It should be noted that after formation of the cavern in the abovedescribed manner, the storage tank 300 may be constructed in another way - such as by the assembly of secant walls, as previously discussed. The cooling unit 452 may then be removed and the freeze pipes 451 left in the ground in the vicinity of the storage tank 300 such that they can be used again in the event of an emergency requiring immediate cooling of the storage tank 300, as previously described.

[0080] Referring now to Figure 4, an alternative system 200’ is now described. The system 200’ is similar to the system 200 described with reference to Figure 3, therefore like reference numerals are used to indicate like parts, with the addition of prime (') in the example described in Figure 4.

[0081] The system 200’ comprises a storage tank 300’ comprising side walls 30T, a floor 302’ and a roof 303’ located below rock and earth 304’. The storage tank 300’ is connected to a safety system 400’ to control leaks from the storage tank 300’ and allow transfer of liquid ammonia into and out of the storage tank 300’. The safety system 400’ comprises a reliquification system 410’, a collection tank 420’, a scrubber 430’ and chimney 43V, a water tank 440’, a backup external cooling system 450’ comprising freeze pipes 45V and an external cooling unit 452’, a liquid ammonia transfer system 460’ and a safety buffer tank 470’.

[0082] The example shown in Figure 4 is arranged to operate in substantially the same way as the example described with reference to Figure 3, except in that the safety buffer tank 470’ provides a tank where overflow ammonia may be stored in the event of overfilling of the storage tank 300’. In this connection, the safety buffer tank 470’ comprises a fluid connection to the storage tank 300’ to allow overflow ammonia to transfer to the safety buffer tank 470’. The safety buffer tank 470’ may further comprise one or more sensors configured to detect ammonia in the safety buffer tank 470’. The one or more sensors may be arranged with suitable electronics to alert an operator of the safety system 200’ of the presence of ammonia in the safety buffer tank 470’ and therefore the overfilling of the storage tank 300’. Optionally, the system 200’ may comprise a control system configured such that on detection of ammonia by the one or more sensors in the safety buffer tank 470’ the filling of the storage tank 300’ is automatically stopped.

[0083] Still referring to Figure 4, it can further be seen that in this example, the system 200’ is arranged in an access road 500’ formed in the ground. In this connection, the storage tank 300’ has been formed as previously described, and the access road 500’ has been excavated as a single deep road from ground level down to the bottom of the cavern during excavation of the cavern and provides a path to remove material during excavation. The access road 500’ is not refilled after the cavern is completely formed, as it is particularly convenient to position the safety system 400’ within the formed access road 500’ after the cavern has been formed and excavation of material through the access road 500’ is complete. After the safety system 400’ has been located within the access road 500’, any remaining free space around the safety system 400’ may be refilled to substantially bury the safety system 400’ in the ground. The described method of using the access road 500’ as a location to position the safety system 400’ avoids the needs to excavate further to provide an additional cavern for the safety system 400’.

[0084] By positioning the described systems 200, 200’ below ground, the risk of fire and explosion is significantly reduced such that no or minimal safety zone above ground may be required. This is particularly advantageous as it allows for the storage of ammonia in city ports where bunkering of ammonia is often conducted, but where many people must live and work in the vicinity thereof. Furthermore, real estate in or near city ports can often be very valuable so it may be advantageous to consume no land area above ground at all when discharge of treated ammonia gas is to sea, or very little land area above ground when discharge of treated ammonia gas is to air.

[0085] In the preferred examples described, it is desirable to keep the ammonia as cold as possible such that boiling is minimised. In this connection, the storage tank 300, 300’ is configured to operate at around atmospheric pressure in normal conditions, and withstand a much higher internal pressure in situations where excessive boiling occurs. In some examples it may be desirable to configure the storage tank 300, 300’ to withstand higher internal pressure. In such examples, the ammonia may be pressurised in normal storage inside the storage tank 300, 300’ and therefore the boiling temperature will be higher than at atmospheric pressure. In this connection, where the ammonia is stored at a pressure of 2.5 bar, for example, the storage tank 300, 300’ may be cooled to only -20°C to maintain sufficiently low boiling within the storage tank 300, 300’. Again, in such cases, the maximum internal pressure that the storage tank 300, 300’ can withstand may need to be increased to provide a safety factor to ensure that the storage tank 300, 300’ does not explode or rupture if the temperature inside the tank is increased, thereby causing an increase in internal pressure due to the boiling of ammonia, as previously described.

[0086] In the described examples the storage tank 300, 300’ does not require insulation to prevent heating from the surroundings.

[0087] It will be understood that the water tanks 440, 440’ of the described systems 200, 200’ may be maintained at a temperature above 0°C, such that they do not freeze. For example, the water tanks 440, 440’ may be provided with insulation and / or heating. Furthermore, the water tanks 440, 440’ may be located distant from the storage tanks 300, 300’, such that the water tanks 440, 440’ can be kept sufficiently warm such that they do not freeze when the storage tank 300, 300’ is maintained at a low temperature. Although not described in detail, it will be understood that operation of the system 200, 200’ may be performed from a remote location. In this connection, the system 200, 200’ may require no personnel nearby. It will be understood that monitoring, control and emergency procedures may all be effected remotely via suitably configured wireless or wired communications systems.

[0088] During operation the storage tank 300, 300’ may store any amount of ammonia up to 100% of the storage capacity of the tank 300, 300’. For practical reasons, it may be desirable to fill the tank to a practical maximum of between 90% and 95% full. In an example where the tank 300, 300’ is at its practical maximum, i.e. around 95% full, there is remaining 5% free space in the tank 300, 300’ which acts as a gas buffer volume.

[0089] In some examples, the storage tank 300, 300’ may not be completely filled. In some examples, the storage tank 300, 300’ may only be filled to, for example, between 70% and 90% of the capacity of the tank 300, 300’ or around 80% of the capacity of the tank 300, 300’. At 80% filled, there is a 20% gas buffer volume. Any boiled ammonia will increase the pressure within the storage tank 300, 300’ more rapidly when there is only 5% gas buffer volume compared to when there is a gas buffer volume of 20% provided. Therefore, the safety of operation of the system 200, 200’ may be improved by providing a larger gas buffer volume.

[0090] In some examples, it may be desirable to operate the external cooling system 450, 450' at all times while there is ammonia in the storage tank 300, 300’. This may provide a further safety barrier since operation of the external cooling system 450, 450’ results in freezing of any ground water surrounding the storage tank 300, 300’. The reaction between ammonia and ice is very slow, therefore if any ammonia leaks from the walls 301 , 30T or floor 302, 302’ of the storage tank 300, 300’ will take a long time to penetrate the surrounding ice.

[0091] In some examples, the storage tank 300, 300’ may comprise a plurality of sensors located around the walls 301 , 30T and floor 302, 302’ of the tank 300, 300’ and configured to detect leaked ammonia.

[0092] In some examples, concrete may also be injected into one or more portions of the system 200, 200’. In some examples, concrete may be injected into the storage tank 300, 300’ for example, but not limited to, the walls 301 , 30T and / or floor 302, 302’. In some examples, concrete may be injected into other components of the system 200, 200’ for example, but not limited to, the backup external cooling system 450, 450’. In some examples, concrete injection may take place during formation / installation of the system 200, 200’ such as during formation / installation of the storage tank 300, 300’. Additionally or alternatively, concrete injection may take place during maintenance of the system such as during maintenance of the storage tank 300, 300’ for example, but not limited to, upon the discovery of cracks or voids in the storage tank 300, 300’. Concrete may then be injected into and provide resealing of components of the system

[0093] 200, 200’ such as, but not limited to, the walls 301 , 30T and / or floor 302, 302’.

Claims

CLAIMS1 . A system (200, 200’) for safely storing ammonia below ground level, comprising: a storage tank (300,300’) located underground for the storage of liquid ammonia; and a safety system (400, 400’) connected to the storage tank (300, 300’) to control leaks from the storage tank (300, 300’) and allow transfer of liquid ammonia into and out of the storage tank (300, 300’), the safety system (400, 400’) comprising: a reliquification system (410, 410’) configured to receive and liquefy gaseous ammonia boiled from the liquid ammonia in the storage tank (300, 300’) and deliver the liquified ammonia to the storage tank (300, 300’); a collection tank (420, 420’) configured to collect leaked liquid ammonia from the safety system (400, 400’) and deliver the leaked liquid ammonia to the storage tank (300, 300’); a scrubber (430, 430’) configured to treat gaseous ammonia leaked from the safety system (400, 400’) and deliver treated gas to the atmosphere or the sea through a carbon filter; a water tank (440, 440’) connected to the scrubber (430, 430’) and configured to receive contaminated water; a backup external cooling system (450, 450’) configured to be operable to provide backup cooling to the storage tank (300, 300’) and collection tank (420, 420’); anda liquid ammonia transfer system (460, 460’) configured to transfer liquid ammonia into and out of the storage tank (300, 300’); wherein the reliquification system (410, 410’) is configured to provide cooling to the storage tank (300, 300’) and the collection tank (420, 420’) as a result of the liquification of ammonia gas.

2. The system (200, 200’) according to claim 1 , wherein the storage tank (300,300’) is configured to withstand up to at least 2 bar internal pressure or at least 3 bar internal pressure or at least 4 bar internal pressure or at least 5 bar internal pressure or at least 10 bar internal pressure.

3. The system (200, 200’) according to claim 2, wherein the reliquification and collection tank (420, 420’) are configured to withstand an internal pressure.

4. The system (200, 200’) according to any preceding claim, wherein the storage tank (300,300’) has a storage capacity of between 1 ,000m3and 40,000m3.

5. The system (200, 200’) according to any preceding claim, wherein the storage tank (300,300’) comprises walls (301 , 30T), a floor (302, 302’) and a roof (303, 303’), wherein the walls (301 , 30T) and / or floor (302, 302’) are made of concrete and the roof (303, 303’) comprises a steelstructure.

6. The system (200, 200’) according to claim 5, wherein the roof (303, 303’) of the storage tank (300,300’) is covered with earth.

7. The system (200, 200’) according to claim 5 or 6, wherein the roof (303, 303’) is welded or bolted to the walls (301 , 30T) and / or floor (302, 302’).

8. The system (200, 200’) according to any preceding claim, wherein the system (200, 200’) further comprises: a first fluid communication conduit fluidly connecting the reliquification system (410, 410’) and the collection tank (420, 420’); and a second fluid communication conduit fluidly connecting the collection tank (420, 420’) and the storage tank (300,300’); wherein the reliquification system (410, 410’) is configured to provide cooling to the first and second fluid communication conduits as a result of the liquification of ammonia gas.

9. The system (200, 200’) according to claim 8, wherein the reliquification system (410, 410’) is configured to provide cooling of the first and second fluid communication conduits such that the first and second fluid communication conduit internal temperatures are between -10 and -35°C or around -20°C.

10. The system (200, 200’) according to any preceding claim, wherein the water tank (440, 440’) has a capacity of between 100m3and 15,000m3.11 . The system (200, 200’) according to any preceding claim, further comprising a chimney (431 , 43T) connected to the scrubber (430, 430’) to allow venting of treated gas to the atmosphere.

12. The system (200, 200’) according to any preceding claim, wherein the reliquification system (410, 410’) is configured to provide cooling of the storage tank (300,300’) such that the storage tank (300,300’) internal temperature is between -5 and -50°C or between -30 and -40°C.

13. The system (200, 200’) according to any preceding claim, wherein the reliquification system (410, 410’) is configured to provide cooling of the collection tank (420, 420’) such that the collection tank (420, 420’) internal temperature is between -10 and -35°C or around -20°C.

14. The system (200, 200’) according to any preceding claim, wherein the backup external cooling system (450, 450’) comprises at least one freeze pipe (451 , 45T) located in the ground sufficiently close to the storage tank (300,300’) such that in use the at least one freeze pipe (451 , 45T) can cool the storage tank (300,300’) such that the storage tank (300,300’)internal temperature is between -5 and -40°C or between -30 and -40°C.

15. The system (200, 200’) according to claim 14, wherein the at least one freeze pipe (451 , 45T) is located in the ground within 2 to 10m or 4 to 8m or around 5m from the storage tank (300,300’).

16. The system (200, 200’) according to claim 14 or 15, wherein the at least one freeze pipe (451 , 45T) located in the ground sufficiently close to the collection tank (420, 420’) such that in use the at least one freeze pipe (451 , 45T) can cool the collection tank (420, 420’) such that the collection tank (420, 420’) internal temperature is between -10 and -35°C or around - 20°C.

17. The system (200, 200’) according to claim 16, wherein the at least one freeze pipe (451 , 451 ’) is located in the ground within 2 to 10m or 4 to 8m or around 5m from the collection tank (420, 420’).

18. The system (200, 200’) according to claim 14 or 15, wherein the at least one freeze pipe (451 , 45T) comprises a plurality of freeze pipes.

19. The system (200, 200’) according to any of claims 14 to 18, wherein the backup external cooling system (450, 450’) further comprises a removable cooling unit (452, 452’) configured to connect to the at least one freezepipe (451 , 45T) to provide cooling of the at least one freeze pipe (451 , 451’).

20. A method of safely storing ammonia underground, comprising the steps of: providing a system (200, 200’) according to any of claims 1 to 19; providing a volume of liquid ammonia in the storage tank (300,300’); operating the reliquification system (410, 410’) to: receive and liquefy gaseous ammonia boiled from the liquid ammonia in the storage tank (300, 300’) and deliver the liquified ammonia to the storage tank (300, 300’); and provide cooling to the storage tank (300, 300’) and the collection tank (420, 420’) as a result of the liquification of ammonia gas.21 . The method of claim 20, further comprising the steps of: collecting leaked liquid ammonia from the safety system (400, 400’) in the collection tank (420, 420’) and delivering the leaked liquid ammonia to the storage tank (300, 300’).

22. The method of claim 20 or 21 , further comprising the steps of: treating gaseous ammonia leaked from the safety system (400,400’) in a scrubber (430, 430’) and delivering treated gas to the atmosphere or the sea through a carbon filter; and collecting contaminated water from the scrubber (430, 430’) in the water tank (440, 440’).

23. The method of any of claims 20 to 22, further comprising the step of: operating the backup external cooling system (450, 450’) to provide cooling to the storage tank (300, 300’) and collection tank (420, 420’).

24. The method of any of claims 20 to 23, further comprising the step of: transferring liquid ammonia into or out of the storage tank (300, 300’) using the liquid ammonia transfer system (460, 460’).

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