Method and system for recovering fuel from an engine

Abstract

A system and method for recovering ammonia from an ammonia-fuelled engine of a vessel. The method comprises: (a) purging the engine with a purge gas to produce an ammonia-containing purge gas stream; (b) passing the ammonia-containing purge gas stream to holding tank(s) on the vessel to create a holding gas; (c) passing the holding gas into a reliquefaction system provided on the vessel to provide a liquid ammonia return stream and a reduced ammonia gaseous stream; and (d) returning the liquid ammonia return stream into a fuel tank configured to supply the engine. The purge gas may be nitrogen. The reliquefaction system may be the same reliquefaction system that the ship uses for recovering boil-off gas, BOG, from a liquefied natural gas, LNG, cargo. There is also described a vessel, such as a liquefied cargo carrier, using said method and system. The engine may be a dual-fuel engine configured to work with a second fuel.

Description

The present invention relates to a method and system recovering ammonia fuel from a shutdown engine for a vessel such as a liquefied gas cargo carrier, as well as a vessel as such. Background Ammonia-fuelled ships are increasingly being developed and built, or ship and marine engines are being adapted to be driven by ammonia-fuel, following a 2018 International Maritime Organisation commitment to cut International shipping’s greenhouse gas emissions. Dual-fuel engines are also more common to assist the transition from using transitional fuels to at least partly using ammonia. Figure 1 shows a schematic layout for a conventional hydrocarbon fuel tank 2 (such as a fuel oil tank or a liquefied petroleum gas (LPG) tank), supplying an engine 8 of a vessel (not shown). The fuel from the fuel tank 2 passes through a conventional fuel supply system 4, generally involving a pump, filters and heat exchangers, to a fuel valve train 6, which supplies the fuel to the engine in a manner known in the art. On ships and vessels with engines running on low-flashpoint fuels, regulations require the engine(s), and piping between fuel valve train(s), to be purged, typically with nitrogen, whenever the engine stops or is shutdown. Purging of the engine is typically required to inert and depressurise the engine and its surrounding piping to make these components safe after stop or shutdown. In Figure 1, purging of the engine can be provided by a purge gas source 12, typically nitrogen. The fuel valve train 6 includes a return train section, that passes vent gasses and liquids to a vent system 14, from which gas or gasses can be vented from a suitable vent mast 16 to atmosphere. During the purging process, some of the fuel will evaporate into the nitrogen purging stream. Where the fuel is a hydrocarbon fuel such as LPG, venting the gas to a vent mast is currently acceptable. However, this is not possible with ammonia, and there is increasing legislation to minimise emissions of ammonia gas, especially in ports and near land, because ammonia is toxic. The present invention seeks to help overcome this problem. Summary According to one aspect of the present invention, there is provided a method of recovering ammonia from an ammonia-fuelled engine of a vessel comprising at least the steps of: (a) purging the engine with a purging gas to produce an ammonia-containing purge gas stream: (b) passing the ammonia-containing purge gas stream to one or more holding tanks on the carrier to create a holding gas; (c) passing the holding gas into a reliquefaction system provided on the vessel to provide a liquid ammonia return stream and a reduced ammonia gaseous stream; and (d) returning the liquid ammonia return stream into an ammonia fuel tank configured to supply the engine. According to a further or independent aspect of the present invention, there is provided a system for of recovering ammonia from an ammonia-fuelled engine of a vessel comprising: (i) a source of purging gas configured to purge the engine with a purging gas to produce an ammonia-containing purge gas stream: (ii) one or more holding tanks on the carrier configured to hold the ammonia-containing purge gas stream and to create a holding gas; (iii) a passageway to pass the holding gas into a reliquefaction system on the vessel to provide a liquid ammonia return stream and a gaseous stream; and (iv) a return passageway configured to return the liquid ammonia return stream into an ammonia fuel tank configured to supply the engine. Typically, the engine is being stopped, shut down or the fuel changed when using the present invention. Optionally, the vessel is a liquefied gas cargo carrier, and the cargo of the liquefied gas cargo carrier is selected from the group comprising: liquefied natural gas (LNG), ethane, a liquefied petrochemical gas such as LPG, propylene, ethylene and ammonia. Thus, optionally, the reliquefaction system is a cargo reliquefaction system for recovering boil-off gas of the cargo. According to a further or independent aspect of the present invention, there is provided a vessel such as a liquefied gas cargo carrier having a reliquefaction system, optionally a cargo reliquefaction system for recovering boil-off gas of the cargo, an ammonia-fuelled engine, and a system or a method as defined herein for recovery of ammonia from the engine. Detailed embodiments and drawings Embodiments of the present invention will now be described by way of example only, and with reference to the accompanying drawings in which: Figure 1 shows schematically a prior art engine and fuel arrangement; Figure 2 shows schematically a prior art liquefied gas cargo tank and dedicated reliquefaction system; Figure 3 shows schematically aspects and embodiments of the present invention both individually and collectively; and Figure 4 shows schematically various arrangements according to the present invention for an engine for vessel, its fuel systems, and a cargo. As discussed above, Figure 1 shows a schematic layout for a conventional hydrocarbon fuel tank 2 such as fuel oil or LPG, supplying an engine 8 of a vessel (not shown). The fuel from the fuel tank 2 passes through a conventional fuel supply system 4, generally involving a pump, filters and heat exchangers, to a fuel valve train 6, which supplies the fuel to the engine in a manner known in the art. Purging of the engine can be provided by a purge gas source 12, typically nitrogen. The fuel valve train 6 includes a return train section that passes the gasses from the engine 8 to a vent system 14, from which vent gas or gasses can be vented from a suitable vent mast 16 to atmosphere. The possible use of ammonia as a fuel in engines has been known for some time. It is now also possible to provide what is termed “green ammonia”, by reacting nitrogen separated from air with hydrogen made by wind or solar-powered water electrolysis. Green ammonia is potentially seen as environmentally friendlier than fossil fuels due to having ‘no-carbon’ emissions from burning. Green ammonia is also a cheaper fuel for the shipping industry than hydrogen, as it is safer, easier to store, and it can be burned in standard internal combustion engines. However, ammonia is also considered to be relatively toxic, and so venting of ammonia is undesirable due to its toxicity, especially near land or in port. Meanwhile, there are regulations that require a ship’s or vessel’s engine, and piping between fuel valve train, to be purged with nitrogen whenever the engine stops, either intentionally or after the engine trips. But purging an ammonia-fuelled engine may lead some of the ammonia fuel being evaporated into the purging nitrogen stream, and thus be vented along with the nitrogen if traditional venting is employed. An example of the present invention provides a method of recovering ammonia from an ammonia-fuelled engine of a vessel, comprising at least the steps of: (a) purging the engine with a purging gas to produce an ammonia-containing purge gas stream: (b) passing the ammonia-containing purge gas stream to one or more holding tanks on the carrier to create a holding gas; (c) passing the holding gas into a reliquefaction system provided on the vessel to provide a liquid ammonia return stream and a reduced ammonia gaseous stream; and (d) returning the liquid ammonia return stream into an ammonia fuel tank configured to supply the engine. The vessel may be any type of form of vessel, typically an ocean-going sea-going vessel. The possible use of partly or fully ammonia-fuelled engines on or for vessels is known in the art, and the use, benefit and operation of ammonia-fuelled engines is not further discussed herein. One type of vessel using or being converted to using partly or fully ammonia-fuelled engines are cargo vessels, typically sea-going and ocean-going vessels. Such vessels can carry any form of cargo, and the present invention is not limited thereby. One type of cargo is a fuel, such as but limited to a liquefied gas. Thus, optionally, the vessel is a liquefied gas cargo carrier. The liquefied gas cargo carrier may be any marine or seagoing vessel, including ships, carriers, barges, etc. Optionally, the cargo of a liquefied gas cargo carrier is selected from the group comprising: liquefied natural gas (LNG), ethane, a liquefied petrochemical gas such as LPG, propylene, ethylene and ammonia. Other liquefied cargoes exist, or may be developed as a liquefied cargo. Such cargos are well known in the art, and generally comprise carrying the cargo at a below ambient temperature. For LNG, this can be below -150°C when measured at 1 atmosphere. Other liquefied cargoes can have boiling points of higher than -110 °C (but still below 0°C) when measured at 1 atmosphere, such as LPG, other liquefied petrochemical gases such as propylene and ethylene, and liquefied ammonia. LPG comprises one or more of propane, n-butane and i-butane, and optionally one or more other hydrocarbons such as propylene, butylene and ethane. Where the vessel is a liquefied gas cargo carrier, boil off gas (BOG) commonly occurs during transportation, so that an on-board method of BOG reliquefaction is typically added to the vessel, to recover and return the BOG back into the cargo tank or tanks. In one embodiment of the present invention, the reliquefaction system is a cargo reliquefaction system for recovering boil-off gas of the cargo. Where the vessel has a cargo that is or includes a fuel, it is also possible for such fuel to be used to partly or fully fuel the engine of the vessel. For example, where the cargo is LPG, LNG, or ammonia, fuel from the engine could be taken directly from one or more cargo tanks and supplied to the engine (via a suitable delivery system or mechanism.). Such arrangements are within the scope of the present invention. Alternatively or additionally, where the vessel has a cargo that is or includes a fuel, it is also possible for such fuel to be used to partly or fully fuel the engine of the vessel, but for such fuel for the engine to be stored separately from the cargo, i.e. in one or more separately designated fuel tanks. Such fuel tanks may be connected with or distinct from the cargo tank(s). That is, irrespective of the nature of the cargo, the vessel may have one or more dedicated fuel tanks containing a fuel which is the same or different to the cargo. Such arrangements are within the scope of the present invention. In one embodiment, the vessel has a non-fuel cargo, and the engine is able to be fuelled by ammonia. In another embodiment, the vessel has a non-fuel cargo, and the engine is able to be fuelled by ammonia and another fuel, such as fuel oil. In another embodiment, the vessel is a liquefied gas cargo carrier, and the engine is able to be fuelled by ammonia. In another embodiment, the vessel is a liquefied gas cargo carrier, and the engine is able to be fuelled by ammonia and another fuel, such as fuel oil. Optionally, the vessel is a liquefied gas cargo carrier. The liquefied gas cargo carrier may be any marine or seagoing vessel, including ships, carriers, barges, etc. In one embodiment of the present invention, the method of recovering ammonia from an ammonia-fuelled engine of a vessel comprises the vessel being a liquefied gas cargo carrier having a cargo reliquefaction system for recovering boil-off gas of the cargo, the method comprising at least the steps of: (a) purging the engine with a purging gas to produce an ammonia-containing purge gas stream: (b) passing the ammonia-containing purge gas stream to one or more holding tanks on the carrier to create a holding gas; (c) passing the holding gas into the cargo reliquefaction system to provide a liquid ammonia return stream and a gaseous stream; and (d) returning the liquid ammonia return stream into an ammonia fuel tank configured to supply the engine. The nature of the engine of the vessel may be an existing or standard internal combustion engine, or an engine that has been adapted or further developed to use ammonia fuel. The term “engine” as used herein applies to one engine or to more than one engine, used to power the vessel through water. Fuel valve trains able to supply fuel to one or more engines are known in the art. The fuel source for the engine is typically one or more fuel tanks working collectively or separately, and the term ‘fuel tank’ as used herein refers to any and all multiple fuel tank arrangements and systems. Where the engine is a dual-fuel engine, the vessel may comprise different fuel tanks for the different fuels. Current ammonia-fuelled engines also generally require a combustion promotor, such as fuel oil or diesel, for ignition support and other known reasons. Optionally, a separate combustion-promotor source or fuel tank is also required. Such combustion support and an associated fuel tank is not further discussed herein. Optionally, the engine is partly, substantially or fully supplied with ammonia fuel from the ammonia fuel tank to which the recovered liquid ammonia return stream is being returned. The purging of engines is known in the art. Purging of the engine is required to inert and depressurise the engine and its surrounding piping to make these components safe after stop or shutdown. Purging gas streams are known in the art, and generally comprise one or more inert gases, including inert to the nature of the engine fuel. A purging gas stream is passed through the engine and any and all associated lines tanks, piping etc. The commonest gas used for a purge gas stream is nitrogen, although other gases may be used or mixed with nitrogen. Optionally, the purging gas is wholly or substantially nitrogen. The method of the present invention involves purging the engine with a purging gas to produce an ammonia-containing purge gas stream. Purging the engine typically happens when the engine is stopped or shutdown. This may occur when the cargo carrier is in low speed forward-reverse manoeuvring conditions, where it is not possible to run the secondary fuel system, or docked, and / or for a change of fuel to the engine. The purging typically involves passing the purging gas through the engine, and optionally also through associated lines, pipework and the piping between fuel valves to be purged, to provide a resultant ammonia-containing purge gas stream, i.e. including ammonia that has evaporated and needs to be purged away from the engine, etc. In the present invention, the ammonia-containing purge gas stream is passed into a stream to one or more holding tanks on the carrier to create a holding gas. The holding gas may include a liquid or liquid portion, but is termed herein as a ‘holding gas’. The holding tank or tanks may be the same or different, and may operate at different parameters, in particular different gas pressures. Optionally, the vessel has at least two or more holding tanks, which may be the same or different. In one embodiment of the present invention, there is provided a first holding tank as a catch tank and a second holding tank as a purge tank. Optionally, any liquid in the one or more holding tanks is recovered to a tank, such as a temporary or permanent tank, such as a fuel tank such as the ammonia fuel tank. Additionally or alternatively, any liquid in the one or more holding tanks is directly or indirectly recovered into a fuel steam for the engine. Optionally, the first holding tank is a liquid holding tank and the second holding tank is a vapour holding tank. A catch tank may be the same or similar to a catch tank used for other engines, optionally having a capacity to contain any liquid portion of the ammonia-containing purge gas stream purged from the known volume of the engine, and optionally the associated pipework and the piping between relevant fuel valves, and to separate any portion of the ammonia-containing purge gas stream being a liquid or in liquid form. As such, the term “ammonia-containing purge gas stream” as used herein includes a stream which includes a liquid portion. Typically, a liquid portion of the ammonia-containing purge gas stream is liquid ammonia, either recovered from the engine, associated pipework and the piping between relevant fuel valves, and / or being condensed from gaseous ammonia on route to the catch tank from the engine. The operating parameters of the catch tank are such as to separate gas and liquid phases of the purge gas from the known volume of the engine, and optionally the associated pipework and the piping between relevant fuel valves. The liquid is retained within the catch tank for later re-injection into the engine. The catch tank is typically pressure-controlled, for example at 25 barg. The vapour from the purge gas is passed to a next tank such as the purge tank. A purge tank may be the same or similar to a purge tank used for other engines, generally having a capacity to contain the volume of ammonia-containing purge gas stream purged from the known volume of the engine, and optionally the associated pipework and the piping between fuel valves. Optionally, the purge tank acts as a buffer tank, and to influence or maintain process conditions, in particularly pressure, in associated portions of the method, such as in the purge tank. In one embodiment of the present invention, the purge tank is influenced by the gaseous pressure in the catch tank, or provides pressure differential to draw vapour out of the catch tank, or both. Optionally, the conditions in the purge tank are variable over time, and operate in conjunction with the other steps of the method of the invention, in particular the amount of volume of purge gas stream being created in step (a). The operating parameters of the purge tank are such as to allow for pressurisation during an engine purging operation. A purge tank can initially be at a pressure of for example 1 barg, with its pressure rising to a maximum, for example of 25 barg, during or after the engine purging operation is completed. Optionally, the size and operating parameters of the one or more holding tanks are configured to be able to hold the vapour or gaseous portion of the ammonia-containing purge gas stream as a holding gas, until further processing. In the method of the present invention, step (c) involves passing the holding gas into the reliquefaction system to provide a liquid ammonia return stream and a gaseous stream. The reliquefaction system may be dedicated to being part of the present invention for recovering ammonia. Alternatively or additionally, an existing cargo reliquefaction system on a vessel, such as for reliquefying a boil off gas (BOG) from a liquefied gas cargo being carried on or in the vessel, can be directly employed to also assist reliquefying ammonia in the holding gas. Such reliquefied ammonia can then be recovered and returned as a liquid ammonia return stream into an ammonia fuel tank configured to supply the engine. Meanwhile, the now reduced-ammonia gaseous purging stream can be further treated and / or vented. The reliquefaction system can be any system, apparatus or arrangement known in the art. Reliquefaction systems, methods, apparatus and plants are well known in the art for reliquefying boil off gas (BOG) from liquefied gas cargoes. Shipboard reliquefaction systems are typically based on the open cycle refrigeration principle of drawing cargo vapour, also known as BOG, from one or more storage tanks of the vessel, and passing the BOG to one or more compressors, in which the BOG is compressed such that the compressed vapour can be cooled and condensed by a coolant. One coolant example is using sea water as the heat sink / refrigerant. By way of example, and referring to the drawings, Figure 2 shows a schematic diagram of a known system for reliquefying boil off gas in a liquefied gas cargo vessel. Such a liquefied gas could be LPG. The LPG is stored in a tank 50 which may be insulated and / or pressurized in order to maintain the petroleum gas in a liquefied state. Vaporization of the LPG in the tank, for instance due to imperfect thermal insulation, will result in the formation of petroleum gas in the overhead space of the tank 50. In order to prevent the build-up of this gas, it is removed from the tank 50 as a boil off gas stream 52. The removed boil off gas 52 is compressed and cooled in a reliquefaction system 54 to condense it before it is returned to the tank 50. In the reliquefaction system 54, the boil off gas stream 52 can be passed through a suction separator 58, and to a compression system 60, which can comprises a first, second and possibly third compression stages, (two shown in Figure 2). The multiple stage compressor 60 produces a compressed discharge stream 62 which can be passed to a condenser 64, in which the compressed discharge stream 62 is cooled against seawater, and possibly other refrigerants. The condenser 64 produces a cooled compressed discharge stream and a warmed seawater stream (not shown). The cooled compressed discharge stream is passed to a discharge stream gas / liquid separation device 68, such as a knock-out drum or accumulator to allow the separation of uncondensed components from the cooled compressed discharge stream. The uncondensed components can be vented through a vent mast 70, while the condensed compressed discharge stream 72 is passed from the gas / liquid separation device 68 for further cooling (not shown) and / or return to the tank 50. The cooled compressed discharge stream 72 may also undergo pressure reduction through an expander or Joule-Thomson valve, where it is expanded, and / or pass through one or more heat exchangers to provide a more condensed stream. In the method of the present invention, passing the holding gas into the reliquefaction system further recovers, by the same liquefaction principle and methodology, ammonia fuel picked up by the purging gas, as a liquid ammonia return stream. The liquid ammonia return stream can then be returned into the or an ammonia fuel tank configured to supply the engine. The resultant gas from the liquefaction process has a reduced-ammonia content, and can be a reduced-ammonia gas stream ready for direct or indirect venting. Thus, the method of the present invention can further provide the step of: (e1) venting the reduced-ammonia gas stream to atmosphere. Additionally or alternatively, the reduced-ammonia gas stream is further treated by one or more further treating processes or steps. One known further treatment step is use of scrubbing, in particular an acid-scrubbing polishing step, intended to further capture or recover ammonia in the gas stream. Thus, the method of the present invention can further provide the step of: (e2) further treating the reduced ammonia gas stream. The present invention also provides a system for of recovering ammonia from an ammonia-fuelled engine of a vessel such as a liquefied gas cargo carrier having a reliquefaction system, for example for recovering boil-off gas of a liquefied gas cargo, comprising: (i) a source of purging gas configured to purge the engine with a purging gas to produce an ammonia-containing purge gas stream: (ii) one or more holding tanks on the carrier configured to hold the ammonia-containing purge gas stream and to create a holding gas; (iii) a passageway to pass the holding gas into the reliquefaction system to provide a liquid ammonia return stream and a reduced ammonia gaseous stream; and (iv) a return passageway configured to return the liquid ammonia return stream into an ammonia fuel tank configured to supply the engine. Optional embodiments of the system of the present invention are discussed above in relation to the discussion of the method of the present invention, and the skilled person can see how such embodiments can be directly applied to the features of the system as described above. For example, in the system of the present invention, the cargo of the liquefied gas cargo carrier is selected from the group comprising: liquefied natural gas (LNG), ethane, a liquefied petrochemical gas such as LPG, propylene, ethylene and ammonia. Optionally, the reliquefaction system is a cargo reliquefaction system for recovering boil-off gas of the cargo. The present invention also provides a vessel having a reliquefaction system, an ammonia-fuelled engine, and a system as defined herein for recovery of ammonia from the engine. The present invention also provides a vessel having a reliquefaction system, an ammonia-fuelled engine, and a method as defined herein to recover ammonia from the engine. Such a vessel may be a liquefied gas cargo carrier, and having an engine being a dual-fuel engine configured to work with a second fuel, such as fuel oil. An embodiment of the present invention is a liquefied gas cargo carrier having a cargo reliquefaction system for recovering boil-off gas of the cargo, an ammonia-fuelled engine, and a system as defined herein for recovery of ammonia from the engine. Another embodiment of the present invention is a liquefied gas cargo carrier having a cargo reliquefaction system for recovering boil-off gas of the cargo, an ammonia-fuelled engine, and a method as defined herein to recover ammonia from the engine. The liquefied gas cargo carrier may be any floating transportation carrier, such as liquefied gas carrier vessels and barges, capable of transporting a variety of cargoes in the liquefied state. The long distance transportation of liquefied gas cargos is typically by oceangoing tankers, having one or more storage tanks to hold the liquefied gas cargo. These storage tanks may be insulated and / or pressurized tanks. Suitable engine or engines for such carriers are known in the art, and include those now using dual-fuel engines that can run on a second fuel, such as a liquefied gas fuel such as LPG, or which are now being tuned to run on ammonia. One such engine is the MAN B&W ME-GI engine produced by MAN Energy Solutions in Germany. Thus, the present invention extends to liquefied gas cargo carriers as defined herein running on first and second fuels, i.e. having an engine being a dual-fuel engine configured to work with ammonia as a second fuel. Referring to the drawings, Figure 3 shows a method of recovering ammonia from an ammonia-fuelled engine 112 of a liquefied gas cargo carrier having a cargo reliquefaction system 114 for recovering boil-off gas of the cargo in a cargo tank 116. The engine 112 may be one engine, or a series of engines, being supplied with fuel in arrangements known in the art. Figure 3 shows a fuel valve train (FVT) 124 having a system or arrangement known in the art for providing a fuel to the engine 112. Typically the FVT 124 has a section being a supply valve train (SVT) 125 ,and section being a return valve train (RVT) 126 in a manner known in the art. The cargo tank 116 includes two low pressure pumps 130 able to pump ammonia as a liquid out of the cargo tank 116 for use as a fuel in the engine 112. The ammonia pumped by the pumps 130 can pass through one or more (two shown) low pressure filters 132, a coriolis meter 134, one or more high pressure pumps 136 (two shown), followed by one or more high pressure filters 138 (two shown) and into the supply valve train 125 of the FVT 124 for use in the engine 112. When it is desired to stop or shut down the engine 112, possibly because the cargo carrier is in a harbour or otherwise docked, or possibly where it is desired to change the fuel source from ammonia to another fuel, or where it is otherwise desired to purge the engine 112, ammonia from the ammonia fuel tank 116 is stopped, and a purging gas 120 from a purging gas source 121 is supplied into the FVT 124 to start purging of the engine 112. Purging of engines is well known in the art to remove gases away from the engine when it stops. Where the gas or gases being removed from an engine are not harmful or toxic, they may be vented directly to atmosphere. However, ammonia is relatively toxic. Thus, a purging gas stream for an ammonia-fuelled engine should not be vented directly to atmosphere. In the example of the present invention shown in Figure 3, purging the engine 112 with a purging gas 120 produces an ammonia-containing purge gas stream 122, which can be passed along a line by closing valve 128 into one or more holding tanks on the cargo carrier to create a holding gas 140. The holding tanks can be one or more suitable tanks able to process and / or hold the ammonia-containing purged gas stream 122 for a time. One embodiment of the holding tanks comprises first having a catch tank 142. The catch tank 142 is at a suitable pressure to allow recovery of any liquid portion of the ammonia-containing purged gas stream 122 as a liquid recovery stream 144, which can then be returned via a suitable line or passageway 129 into a or the feed for the engine 112 when an ammonia fuel feed to the engine is next required. The catch tank 142 may be the same or similar to a catch tank used for other engines. The catch tank 142 can have a capacity to contain the volume of ammonia-containing purge gas stream purged from the known volume of the engine, and optionally the associated pipework and the piping between relevant fuel valves, and to separate any portion of the ammonia-containing purge gas stream being a liquid or in liquid form. The pressure in the catch tank 142 can be maintained at a minimum pressure to ensure that any liquid therein can be directly or easily reinjected into the engine 112. A nitrogen source 146 can ensure or maintain a minimum pressure or flow of gas through the catch tank 142. The gas steam 148 from the catch tank 142 can pass to a purge tank 152. A purge tank can prevent the pressure in a catch tank from rising above a maximum operating pressure, for example, of 25 barg. In the present invention, the purge tank 152 can be at a suitable pressure to achieve a pressure balance between at least the subsequent re-liquefaction system 114 and the catch tank 142. Meanwhile, the conventional intention of the cargo re-liquefaction system 114 in Figure 3 is to take the boil-off gas 160 from the cargo tank 116, and wholly or substantially re-liquefy the boil-off gas to provide a return boil-off gas stream 150 to be returned to the cargo tank 116 in a manner known in the art. In the present invention, the holding gas 140 can also be passed into the cargo reliquefaction system 114 to provide a liquid ammonia return stream 150 and a reduced ammonia gaseous stream 154. The holding gas 140 can pass through some or all of the same parts or components or processes of the cargo reliquefaction system 114 needed to re-liquefy gaseous ammonia or ammonia vapour as a boil off gas 160 from the tank 116 in a manner known in the art. Figure 3 schematically shows the cargo reliquefaction system 114 comprising a suction separator 142, a nominal compressor 144, a nominal cooler 146, and a nominal gas I liquid separator 148. The compressor 144 may comprise one or more compressors, typically in a train, able to compress a gaseous flow to a higher pressure in a manner known in the art. The cooler 146 may also comprise one or more heat exchangers, economisers, condensers, coolers, etc., known in the art, able to cool the discharge stream from the compressor 144 using suitable coolants or refrigerants. Suitable coolants and refrigerants include sea water and known refrigerants such as SMR or the like, none of which are shown in detail in Figure 3. The compressor 144 may also comprise one or more heat exchangers, economisers, condensers, coolers, etc., known in the art, able to cool an interstage discharge stream in a multi-stage compressor arrangement, using suitable coolants or refrigerants, one or more of which may be provided from subsequent compressed streams. Cargo reliquefaction systems, plants, apparatus and other arrangements are well known in the art, especially for liquefied gas BOG reliquefaction. Examples include the systems shown in WO2012 / 136991, WO2012 / 143699, WO2017 / 144919 and WO2018 / 193244, which are incorporated herein by reference. The cooled stream from the cooler 146 is then passed into a gas / liquid separator 148. The person skilled in the art is aware of suitable gas / liquid separators, optionally involving additional cooling in a manner known in the art, to provide a gaseous stream 154 and a liquid stream, which can return into the cargo tank 116 along the line 150. Optionally, the gaseous stream 154 produced from the gas / liquid separator 148, when the input gas stream into the cargo reliquefaction system 114 is via line 140, is now either ammonia-free or sufficiently ammonia-free to be able to be vented to atmosphere through a vent mast 156. Additionally or alternatively, the ammonia-gaseous stream passes into a treatment process (not shown) for further reduction of the ammonia content. The process may involve the use of acid scrubbing or the like, able to clean or separate or otherwise absorb ammonia content in a manner known in the art, such that the vent gas is further ‘cleaned’ of is ammonia content and is ready to be vented to atmosphere through the vent mast 156. Figure 3 shows an example of the present invention able to condense the ammonia portion of a purging gas stream passing through an ammonia-fuelled engine, and recovery of such ammonia back into an ammonia fuel tank, using an existing cargo reliquefaction system. The use of one or more holding tanks allows space and time for initial liquid ammonia recovery. Figure 3 also shows the cargo tank 116 acting as both a general cargo tank for a liquefied ammonia a cargo, and as a fuel tank for the engine. However, the engine may also be a dual-fuel engine configured to work with another fuel such as fuel oil. Figure 4 shows more schematically an arrangement or system wherein an engine 210 which can be fuelled by ammonia from a dedicated ammonia fuel tank 212. The ammonia can pass into a suitable fuel supply system 216 and into a fuel valve train (FVT) 218 for supply into the engine 210 in a manner known in the art. The engine 210 can power a vessel transporting a liquefied gas cargo in one or more cargo tanks 230. Boil off gas 232 from the liquefied gas in the cargo tanks 230 can be reliqufied by a reliquefaction system 240 for recovering boil-off gas 232 as a liquefied stream 234 back into the cargo tanks 230. In the example of the present invention shown in Figure 4, purging the engine 210 with a purging gas 220 from a source 222 into the FVT 218 produces an ammonia-containing purge gas stream 242, which can be passed along a line into one or more holding tanks 244 on the vessel to create a holding gas 246. Thus, Figure 4 shows an example of a method of recovering ammonia from an ammonia-fuelled engine 210 of a vessel having a reliquefaction system 240, comprising at least the steps of: (a) purging the engine 210 with a purging gas 220 to produce an ammonia-containing purge gas stream 242: (b) passing the ammonia-containing purge gas stream 242 to one or more holding tanks 244 on the carrier to create a holding gas 246; (c) passing the holding gas 246 into the reliquefaction system 240 to provide a liquid ammonia return stream 236 and a reduced ammonia gaseous stream 238; and (d) returning the liquid ammonia return stream 236 into an ammonia fuel tank 212 configured to supply the engine 210. Figure 4 also shows a system for of recovering ammonia from an ammonia-fuelled engine 210 of a vessel having a reliquefaction system 240, comprising: (i) a source of purging gas 222 configured to purge the engine with a purging gas 220 to produce an ammonia-containing purge gas stream 242: (ii) one or more holding tanks 244 on the carrier configured to hold the ammonia-containing purge gas stream 242 and to create a holding gas 246; (iii) a passageway 246 to pass the holding gas 246 into the reliquefaction system 240 to provide a liquid ammonia return stream 236 and a gaseous stream 238; and (iv) a return passageway 236 configured to return the liquid ammonia return stream 236 into an ammonia fuel tank 212 configured to supply the engine 210. Figure 4 shows recovery of the liquid portion of the ammonia-containing purge gas stream 242 as a return stream 248 back into the fuel supply system 216 when the engine is next using ammonia fuel. Figure 4 also shows in step (c) or step (iii) of the present invention, an example of the present invention involving passing the holding gas 250 into a gas portion of the ammonia fuel tank 212, and passing gas 252 from the ammonia fuel tank 212 to the reliquefaction system 240. The skilled reader can see how this could be adapted to 5 pass the holding gas 250 into a liquid portion of the ammonia fuel tank 212, and passing gas 252 from the ammonia fuel tank to the reliquefaction system 240. The present invention shows various methods, systems and apparatus for recovering ammonia from a purge stream for an ammonia-fuelled engine of a vessel 10 so as to overcome or reduce the problem of venting ammonia to atmosphere.

Claims

1. A method of recovering ammonia from an ammonia-fuelled engine of a vessel comprising at least the steps of:(a) purging the engine with a purging gas to produce an ammonia-containing purge gas stream:(b) passing the ammonia-containing purge gas stream to one or more holding tanks on the carrier to create a holding gas;(c) passing the holding gas into a reliquefaction system provided on the vessel to provide a liquid ammonia return stream and a reduced ammonia gaseous stream; and(d) returning the liquid ammonia return stream into an ammonia fuel tank configured to supply the engine.

2. A method as claimed in claim 1 wherein the purging gas is wholly or substantially nitrogen.

3. A method as claimed in claim 1 or claim 2 wherein the engine is wholly or substantially supplied with ammonia fuel from the ammonia fuel tank.

4. A method as claimed in any one of the preceding claims wherein the vessel is a liquefied gas cargo carrier.

5. A method as claimed in claim 4 wherein the cargo of the liquefied gas cargo carrier is selected from the group comprising: liquefied natural gas (LNG), ethane, a liquefied petrochemical gas (LPG), propylene, ethylene and ammonia.

6. A method as claimed in claim 4 or claim 5 wherein the reliquefaction system is a cargo reliquefaction system for recovering boil-off gas of the cargo.

7. A method as claimed in any one of the preceding claims wherein the engine is a dual-fuel engine configured to work with a second fuel.

8. A method as claimed in any one of the preceding claims having two or more holding tanks.

9. A method as claimed in claim 8 having a first holding tank as a catch tank and a second holding tank as a purge tank.

10. A method as claimed in any one of the preceding claims wherein any liquid in the one or more holding tanks is recovered to the ammonia fuel tank.

11. A method as claimed in any one of the preceding claims wherein step (c) comprises passing the holding gas through the ammonia fuel tank, and passing gas from the ammonia fuel tank to the reliquefaction system.

12. A method as claimed in claim 11 wherein step (c) comprises passing the holding gas into a liquid portion of the ammonia fuel tank, and passing gas from the ammonia fuel tank to the reliquefaction system.

13. A method as claimed in claim 11 wherein step (c) comprises passing the holding gas into a gas portion of the ammonia fuel tank, and passing gas from the ammonia fuel tank to the reliquefaction system.

14. A method as claimed in any one of the preceding claims further providing the step of:(e1) venting the reduced ammonia gaseous stream to atmosphere,15. A method as claimed in any one of the preceding claims further providing the step of:(e2) further treating the reduced ammonia gaseous gas stream.

16. A system for of recovering ammonia from an ammonia-fuelled engine of a vessel comprising:(i) a source of purging gas configured to purge the engine with a purging gas to produce an ammonia-containing purge gas stream:(ii) one or more holding tanks on the carrier configured to hold the ammonia-containing purge gas stream and to create a holding gas;(iii) a passageway to pass the holding gas into a reliquefaction system on the vessel to provide a liquid ammonia return stream and a gaseous stream; and(iv) a return passageway configured to return the liquid ammonia return stream into an ammonia fuel tank configured to supply the engine.

17. A system as claimed in claim 16 wherein the vessel is a liquefied gas cargo carrier.

18. A system as claimed in claim 17 wherein the cargo of the liquefied gas cargo carrier is selected from the group comprising: liquefied natural gas (LNG), ethane, a liquefied petrochemical gas (LPG), propylene, ethylene and ammonia.

19. A system as claimed in claim 17 or claim 18 wherein the reliquefaction system is a cargo reliquefaction system for recovering boil-off gas of the cargo.

20. A vessel having a reliquefaction system, an ammonia-fuelled engine, and a system as defined in any one of claims 16 to 18 for recovery of ammonia from the engine.

21. A vessel having a reliquefaction system, an ammonia-fuelled engine, and a method as defined in any one of claims 1 to 15 to recover ammonia from the engine.

22. A vessel as claimed in claim 20 or claim 21 being a liquefied gas cargocarrier, and having an engine being a dual-fuel engine configured to work with a second fuel.AMENDMENTS TO THE CLAIMS HAVE BEEN FILED AS FOLLOWS:08 04 25CLAIMS1. A method of recovering ammonia from an ammonia-fuelled engine of a vessel comprising at least the steps of:(a) purging the engine with a purging gas to produce an ammonia-containing purge gas stream:(b) passing the ammonia-containing purge gas stream to one or more holding tanks on the vessel to create a holding gas;(c) passing the holding gas into a reliquefaction system provided on the vessel to provide a liquid ammonia return stream and a reduced ammonia gaseous stream; and(d) returning the liquid ammonia return stream into an ammonia fuel tank configured to supply the engine.

2. A method as claimed in claim 1 wherein the purging gas is wholly or substantially nitrogen.

3. A method as claimed in claim 1 or claim 2 wherein the engine is wholly or substantially supplied with ammonia fuel from the ammonia fuel tank.

4. A method as claimed in any one of the preceding claims wherein the vessel is a liquefied gas cargo carrier.

5. A method as claimed in claim 4 wherein the cargo of the liquefied gas cargo carrier is selected from the group comprising: liquefied natural gas (LNG), ethane, a liquefied petrochemical gas (LPG), propylene, ethylene and ammonia.

6. A method as claimed in claim 4 or claim 5 wherein the reliquefaction system is a cargo reliquefaction system for recovering boil-off gas of the cargo.

7. A method as claimed in any one of the preceding claims wherein the engine is a dual-fuel engine configured to work with a second fuel.08 04 258. A method as claimed in any one of the preceding claims having two or more holding tanks.

9. A method as claimed in claim 8 having a first holding tank as a catch tank 5 and a second holding tank as a purge tank.

10. A method as claimed in any one of the preceding claims wherein any liquid in the one or more holding tanks is recovered to the ammonia fuel tank.10 11. A method as claimed in any one of the preceding claims wherein step (c)comprises passing the holding gas through the ammonia fuel tank, and passing gas from the ammonia fuel tank to the reliquefaction system.

12. A method as claimed in claim 11 wherein step (c) comprises passing the 15 holding gas into a liquid portion of the ammonia fuel tank, and passing gas from the ammonia fuel tank to the reliquefaction system.

13. A method as claimed in claim 11 wherein step (c) comprises passing the holding gas into a gas portion of the ammonia fuel tank, and passing gas from the 20 ammonia fuel tank to the reliquefaction system.

14. A method as claimed in any one of the preceding claims further providing the step of:(e1) venting the reduced ammonia gaseous stream to atmosphere,2515. A method as claimed in any one of the preceding claims further providing the step of:(e2) further treating the reduced ammonia gaseous gas stream.30 16. A system for of recovering ammonia from an ammonia-fuelled engine of avessel comprising:(i) a source of purging gas configured to purge the engine with a purging gas to produce an ammonia-containing purge gas stream:(ii) one or more holding tanks on the vessel configured to hold the ammonia-35 containing purge gas stream and to create a holding gas;08 04 25(iii) a passageway to pass the holding gas into a reliquefaction system on the vessel to provide a liquid ammonia return stream and a gaseous stream; and(iv) a return passageway configured to return the liquid ammonia return stream into an ammonia fuel tank configured to supply the engine.

17. A system as claimed in claim 16 wherein the vessel is a liquefied gas cargo carrier.

18. A system as claimed in claim 17 wherein the cargo of the liquefied gas cargo carrier is selected from the group comprising: liquefied natural gas (LNG), ethane, a liquefied petrochemical gas (LPG), propylene, ethylene and ammonia.

19. A system as claimed in claim 17 or claim 18 wherein the reliquefaction system is a cargo reliquefaction system for recovering boil-off gas of the cargo.

20. A vessel having a reliquefaction system, an ammonia-fuelled engine, and a system as defined in any one of claims 16 to 18 for recovery of ammonia from the engine.

21. A vessel having a reliquefaction system, an ammonia-fuelled engine, and able to recover ammonia using a method as defined in any one of claims 1 to 15.

22. A vessel as claimed in claim 20 or claim 21 being a liquefied gas cargo carrier, and having an engine being a dual-fuel engine configured to work with a second fuel.

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