Method and system for recovering fuel from an engine

Abstract

There is described a method of recovering fuel, such as ammonia, and a ventable stream from a purging stream purging an engine 112 of a sea-going vessel, comprising: (a) purging the engine with a purg

Description

The present invention relates to a method and system recovering 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 traditional 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, both for ammonia-fuelled engines, as well as other fuelled engines where emissions may be or may become problematic after an engine shut down. Summary According to one aspect of the present invention, there is provided a method of recovering fuel and a ventable stream from a purging stream purging an engine of a sea-going vessel, the vessel having fuel in a low-temperature fuel tank configured to supply the engine, comprising at least the steps of: (a) purging the engine with a purging gas to produce an fuel-containing purge gas stream; (b) passing the fuel-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 low-temperature recovery heat exchanger; (d) passing a cooling stream of fuel provided from the fuel tank into the low-temperature recovery heat exchanger to cool the holding gas and to provide a warmer cooling stream, and a cooler holding gas stream; (e) passing the cooler holding gas stream into a separator to provide a gaseous reduced-fuel stream and a liquid fuel stream; and (f) treating the gaseous reduced-fuel stream to further reduce the fuel percentage in the gaseous reduced-fuel stream, and to provide a ventable stream. Optionally, the method further comprises the step of: (g) passing the warmer cooling stream into a reliquefaction system or a refrigeration system provided on the vessel to provide a liquid fuel return stream, and returning the liquid fuel return stream into the fuel tank. According to a further or independent aspect of the present invention, there is provided a system for recovering fuel and a ventable stream from a purging stream purging an engine of a sea-going vessel, the vessel having fuel in a low-temperature fuel tank configured to supply the engine, comprising: (i) a source of purging gas configured to purge the engine with a purging gas to produce a fuel-containing purge gas stream: (ii) one or more holding tanks on the vessel configured to hold the fuelcontaining purge gas stream and to create a holding gas; (iii) a low-temperature recovery heat exchanger; (iv) a passageway to pass the holding gas into the low-temperature recovery heat exchanger; (v) a passageway to pass a cooling stream of fuel from the fuel tank into the low-temperature recovery heat exchanger to cool the holding gas and to provide a warmer cooling stream, and a cooler holding gas stream; (vi) a separator to separate the cooler holding gas stream into a gaseous reduced-fuel stream and a liquid fuel stream; and (vii) a treatment unit to treat the gaseous reduced-fuel stream to further reduce the fuel percentage in the gaseous reduced-fuel stream and to provide a ventable stream. 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; Figures 2 to 4 show schematically aspects and embodiments of the present invention both individually and collectively; and Figure 5 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 of recovering fuel and a ventable stream from a purging stream purging an engine of a sea-going vessel, the vessel having fuel in a low-temperature fuel tank configured to supply the engine, comprising at least the steps of: (a) purging the engine with a purging gas to produce an fuel-containing purge gas stream: (b) passing the fuel-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 low-temperature recovery heat exchanger; (d) passing a cooling stream of fuel provided from the fuel tank into the low-temperature recovery heat exchanger to cool the holding gas and to provide a warmer cooling stream, and a cooler holding gas stream; (e) passing the cooler holding gas stream into a separator to provide a gaseous reduced-fuel stream and a liquid fuel stream; and (f) treating the gaseous reduced-fuel stream to further reduce the fuel percentage in the gaseous reduced-fuel stream, and to provide a ventable stream. The sea-going vessel may be any type of form of vessel, typically an ocean-going or other sea-going vessel, including vessels intended for coastal or harbour use. All such vessels include at least one engine for motion, and a fuel tank to supply fuel to such engine or engines. Hydrocarbon-fuelled engines for cargo ships and vessels are well known in the art. The possible use of partly or fully ammonia-fuelled engines on or for vessels is also 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. 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 vessel includes a reliquefaction system for recovering boil-off gas of the cargo. In another embodiment of the present invention, the method further comprises the step of: (g) passing the warmer cooling stream into a reliquefaction system or a refrigeration system provided on the vessel to provide a liquid fuel return stream, and returning the liquid fuel return stream into the fuel tank. 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. In another embodiment, the vessel is a liquefied gas cargo carrier, and the engine is able to be fuelled by a hydrocarbon fuel, optionally being the liquefied gas cargo. In another embodiment, the vessel is a liquefied gas cargo carrier, and the engine is able to be fuelled by a hydrocarbon fuel, optionally being the liquefied gas cargo, and ammonia. Optionally, the fuel in the method of the present invention is one or more of the group comprising: ammonia, liquefied petroleum gas, methane, ethane, propane, butane and dimethyl ether (DME); preferably ammonia. 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. According to one aspect of the present invention, there is provided a method of recovering ammonia and a ventable stream from a purging stream purging an engine of a sea-going vessel, the vessel having fuel in a low-temperature fuel tank configured to supply the engine, 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 low-temperature recovery heat exchanger; (d) passing a cooling stream of fuel provided from the fuel tank into the low-temperature recovery heat exchanger to cool the holding gas and to provide a gaseous cooling stream, and a cooler holding gas stream; (e) passing the cooler holding gas stream into a separator to provide a gaseous reduced-ammonia stream and a liquid ammonia stream; and (f) treating the gaseous reduced-ammonia stream to further reduce the ammonia percentage in the gaseous reduced-ammonia stream, and to provide a ventable stream. 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. 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 fuel from a fuel tank to which any recovered liquid 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 a fuel-containing purge gas stream. Purging the engine typically happens when the engine is stopped or shutdown. This may occur when the sea-going vessel 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 fuel-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 fuel-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. Optionally, any liquid in the one or more holding tanks is recovered to a tank, such as a temporary or permanent tank, such as the 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, one holding tank is a catch 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 fuel-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 fuel-containing purge gas stream being a liquid or in liquid form. As such, the term “fuel-containing purge gas stream” as used herein includes a stream which includes a liquid portion. Typically, a liquid portion of the fuelcontaining purge gas stream is liquid fuel such as liquid ammonia, either recovered from the engine, associated pipework and the piping between relevant fuel valves, and / or being condensed from gaseous fuel 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 22-25 barg. The low-temperature recovery heat exchanger may be any suitable heat exchanger, such as but not limited to having a shell side and a coil. The low-temperature is generally be below 0°C, optionally in the range -5°C to - 50°C and including in the range -15°C to -30°C, such as and including working temperatures such as -10°C, 15°C, -20°C, -25°C, -30°C, -35°C and lower. In one embodiment of the present invention, this low-temperature is provided by the cooled nature of the fuel from the low-temperature fuel tank which will be at or near the same type of low-temperature. The cooling stream of fuel for the low-temperature heat exchanger can be provided directly or indirectly from the fuel tank. That is, the cooling stream of fuel can be provided directly from the fuel tank, or from an outlet line or stream from the fuel tank designed for passing or providing fuel to another apparatus, unit or part of the vessel. For example, an outlet line or stream from the fuel tank for passing the fuel stream to the engine of the vessel. The cooling stream of fuel can be processed for the low-temperature heat exchanger after being provided from the fuel tank. For example, the cooling stream can be filtered, and / or undergo a pressure change, and / or undergo a temperature change, prior to passing into the low-temperature heat exchanger. For example, the cooling stream can be altered, such as cooled, prior to passing into the low-temperature heat exchanger. The cooling stream of fuel provided from the fuel tank provides a coolant or cooling action to one part, typically one side, such as the shell side, of the heat exchanger to cool the holding gas. Optionally, the holding gas is provided into the heat exchanger on the ‘other’ side, typically being in the nature of a coil, to exchange heat with the cooling stream of fuel. Due to the cooling action of the cooling stream of fuel in the heat exchanger, the heat exchanger is able to cool the incoming holding gas to provide a warmer cooling stream, and cooler holding gas stream. The warmer cooling stream can be a warmer gaseous cooling stream, or a warmer liquid cooling stream, or both. The warmer cooling stream from the heat exchanger is a fuel stream, which can used as fuel, or reliqufied in a manner described herein if a gaseous stream, using a reliquefaction system as described herein, optionally for return to the fuel tank as a liquid stream. Meanwhile, the cooler holding gas stream typically will be a mixture of liquid, having a higher or richer amount of fuel relative to the holding gas, and vapour having a lower or leaner or reduced amount of fuel. In the next step of the present invention, the cooler holding gas stream passes into a separator. The separator may be or have any form or type or design known in the art, so as to provide a gaseous stream at or near the top of the separator, and a liquid stream at or near the bottom of the separator. In the present invention, the low-temperature heat exchanger and separator may be separate units, or be wholly or substantially integral. Thus, the passing of the cooler holding gas stream from the heat exchanger to the separator may occur externally or internally. In the present invention, the separator provides a gaseous reduced-fuel stream, which stream typically has a lower, leaner or reduced amount or percentage of fuel therein, and thus a higher or richer amount of purge gas therein, e.g. a ‘fuel-poor’ stream. It is now easier and more efficient to treat such a stream, to wholly or substantially reduce the fuel content thereof prior to venting the remainder of the stream, (being now only purge gas). Meanwhile, the separator also provides a liquid stream which typically has a lower, leaner or reduced amount or percentage of purge gas therein, and thus a higher or richer amount of fuel therein, e.g. a ‘fuel-rich’ stream. This stream is now useable in the low-temperature heat exchanger as additional coolant, prior to becoming part of the warmer cooling stream from the heat exchanger, (and thus then reusable as fuel, and / or reliquifyable in a manner described herein if a gaseous stream , using a reliquefaction system as described herein, optionally for return to the fuel tank as a liquid stream). The next step of the present invention is to treat the gaseous reduced-fuel stream to further reduce the fuel percentage in the gaseous reduced-fuel stream, and to provide a ventable stream, i.e. a stream able to be vented to the ambient or atmosphere, depending on local emission regulations. Optionally, the ventable stream is directly ventable to atmosphere or the ambient environment when allowed, or may undergo one or more further treatments until it is suitable for local emission regulations. Such treatments include but are not limited to known abatement techniques, catalytic reduction, burning, scrubbing, air dilution, acid treatments, water baths, etc. Such treatment or treatments will be adapted to suit the nature of the fuel. Optionally, treating the gaseous reduced-fuel stream in step (f) comprises passing the gaseous reduced-fuel stream through one or more of the group comprising: a water bath, a catalytic reducer, a scrubber, an air diluter, a burner, or a combination of same. One treatmentfor the gaseous reduced-fuel stream is to use one or more suitable water baths, generally comprising a suitable bath of water having the property of being able to absorb fuel following the passage of the gaseous reduced-fuel stream therethrough. It is known in the art that water is suitable to absorb ammonia in a gaseous stream passing therethrough, and the present invention includes the embodiment of treating the gaseous fuel stream using a water bath in order to absorb the fuel, such fuel optionally being ammonia. Another treatment for the gaseous reduced-fuel stream is to use a catalytic reducer, such as a selective catalytic reducer. Catalytic reduction using a catalyst to covert the or any ammonia gas still in the gaseous reduced-fuel stream into water and nitrogen, in a method known in the art. The sea-going vessel may have a catalytic reducer connected to the engine to help treat engine exhaust gas or gases, and optionally, the present invention could use such an existing catalytic reducer. Another treatment for the gaseous reduced-fuel stream is to burn the gaseous reduced-fuel stream in a suitable burner Another treatment for the gaseous reduced-fuel stream is to treat the gaseous reduced-fuel stream in a suitable scrubber. The present invention provides for one or more treatments of the gaseous reduced-fuel stream in series or in parallel. In one embodiment, step (f) comprises passing a portion of the gaseous reduced-fuel stream through a water bath, and passing a portion of the gaseous reduced-fuel stream through a catalytic reducer. The water bath provides an effluent from its treatment of the gaseous reduced-fuel stream, and the method could further provide wherein effluent from the water bath is used in the catalytic reduction of the gaseous reduced-fuel stream. For example, where the effluent is ammonium hydroxide, created by the adsorption and reaction of ammonia with the water in the water bath, such ammonium hydroxide could be used as a reductant in the catalytic reducer. Optionally, the method of the present invention further comprises holding the gaseous reduced-fuel stream in a post-separator holding tank prior to step (f). Such a holding tank could be suitable for holding both the gaseous reduced-fuel stream and any other gases or streams on the sea-going vessel that require ammonia-reduction treatment. Such a holding tank also allows for flow organisation and timing into the one or more processes used to provide the treatment of step (f). Optionally, the method of the present invention further comprises compressing the gaseous reduced-fuel stream prior to step (f). Such compression allows a greater volume of gaseous reduced-fuel stream to be accommodated by the one or more processes used to provide the treatment of step (f), and / or any post-separator holding tank as described herein. By using a suitable selection of the one or more processes used to provide the treatment of step (f), and optionally a holding tank and pre-compression of the gaseous reduced-fuel stream, the vessel can suitably size the apparatus needed for step (f), and / or suitably time the one or more processes used to provide the treatment of step (f). In this way, the present invention can more advantageously provide the treatment of step (f), possibly in a shorter timescale, possibly in combination with treating other gases or streams on the vessel, and / or in relation to the volume of streams or gases to be treated, and / or in relation to the location of the treatment of step (f) or venting of the ventable stream. Some locations are unsuitable for the discharge of certain gases into the atmosphere, and the present invention can advantageously seek to reduce or avoid the need for venting of the ventable stream until a suitable location for the sea-going vessel is achieved. As described above, the liquid fuel stream provided by the separator may be returned to a fuel tank, or for use in the engine, or may be used as part of the cooling stream required to pass into the low-temperature recovery heat exchanger, to provide cooling of the holding gas in a manner described above. That is, the liquid fuel stream may provide a suitable source or resource in combination with the cooling stream of fuel from the fuel tank, into the low-temperature recovery heat exchanger. Optionally, the holding gas passes through one or more oil-coalescers and one or more oil-absorbers, to reduce oil content of the holding gas in a manner known in the art. In an optional embodiment of the method of the present invention, step (g) involves passing the warmer cooling stream into a reliquefaction system or a refrigeration system provided on the vessel to provide a liquid fuel return stream, and returning the liquid fuel return stream into the fuel tank. As mentioned hereinbefore, the reliquefaction system may be dedicated to being part of the present invention for recovering fuel. 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 fuel in the warmer cooling stream where it is a gaseous stream. 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. Such reliquefied fuel can then be recovered and returned as a liquid fuel return stream into a fuel tank configured to supply the engine. Meanwhile, the now reduced-fuel 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, a known system for reliquefying boil off gas in a liquefied gas cargo vessel and the liquefied gas could be LPG. The LPG can be stored in a relatively low pressure tank 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. In order to prevent the build-up of this gas, it is removed from the tank as a boil off gas stream. The removed boil off gas is compressed and cooled in a reliquefaction system to condense it before it is returned to the tank. In the reliquefaction system, the boil off gas stream can be passed through a suction separator, and to a compression system, which can comprise first, second and possibly third compression stages. The multiple stage compressor produces a compressed discharge stream which can be passed to a condenser, in which the compressed discharge stream is cooled against seawater, and possibly other refrigerants. The condenser 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, 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, while the condensed compressed discharge stream is passed from the gas / liquid separation device for further cooling (not shown) and / or return to the tank. The cooled compressed discharge stream 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 warmer cooling stream into the reliquefaction system or a refrigeration system further recovers, by the same liquefaction principle and methodology, fuel picked up by the purging gas, as a liquid fuel return stream. The liquid return stream can then be returned into the or a fuel tank configured to supply the engine. Optionally, the method the present invention further provides the purging gas is wholly or substantially nitrogen. Optionally, the method of the present invention further provides that the sea-going vessel is a liquefied gas cargo carrier, and the cargo of the liquefied gas cargo carrier is selected from the group comprising: ammonia, liquefied petroleum gas, methane, ethane, propane, butane and dimethyl ether (DME); preferably ammonia. Optionally, the method of the present invention further provides that any reliquefaction system involved is a cargo reliquefaction system for recovering boil-off gas of the cargo. Optionally, the method of the present invention further provides that the fuel tank of the vessel is a low temperature fuel tank. Optionally, the method of the present invention further provides wherein the engine is a dual-fuel engine configured to also work with a second fuel. Optionally, the method of the present invention further comprises the step of passing the holding gas through one or more oil filters prior to step (c), so as to reduce the oil content of the holding gas prior to the low-temperature heat exchanger. Oil is not desired in low-temperature processes in case such oil solidifies and clogs pipework. Optionally, the method of the present invention further provides the step of recovering any liquid in the low temperature heat exchanger back to the fuel tank. Optionally, the method of the present invention further provides wherein step (g) comprises passing the gaseous reduced-fuel stream through one or more of the group comprising: a water bath, a catalytic reducer, a scrubber, an air diluter, a burner, or a combination of same. Optionally, the method of the present invention further comprises holding the gaseous reduced-fuel stream in a post-separator holding tank prior to a catalytic reducer or a scrubber or a burner or an air diluter. Optionally, step (f) of the method comprises passing a portion of the gaseous reduced-fuel stream through a water bath, and passing a portion of the gaseous reduced-fuel stream through a catalytic reducer. Optionally, effluent from the water bath is used in the catalytic reduction of the gaseous reduced-fuel stream. Optionally, the method further comprises compressing the gaseous reduced-fuel stream prior to step (f). Optionally, step (f) of the method comprises passing the gaseous reduced-fuel stream through a selective catalytic reducer connected to the engine of the seagoing vessel. Optionally, the method of the present invention further provides the step of recovering the liquid fuel stream from the separator to the low temperature heat exchanger. In this way, the coolant required in the low-temperature heat exchanger can be supplied by the recovered the liquid fuel stream, to reduce the requirement for fuel from the fuel tank to be the coolant. Optionally, the method / of the present invention further provides the pressure of the holding gas being in the range 15-30 barg, such as in the range 20-25 barg, such as 22 barg. The present invention also provides a system for recovering fuel and a ventable stream from a purging stream purging an engine of a sea-going vessel, the vessel having fuel in a low-temperature fuel tank configured to supply the engine, comprising: (i) a source of purging gas configured to purge the engine with a purging gas to produce a fuel-containing purge gas stream; (ii) one or more holding tanks on the vessel configured to hold the fuelcontaining purge gas stream and to create a holding gas; (iii) a low-temperature recovery heat exchanger; (iv) a passageway to pass the holding gas into the low-temperature recovery heat exchanger; (v) a passageway to pass a cooling stream of fuel from the fuel tank into the low-temperature recovery heat exchanger to cool the holding gas and to provide a warmer cooling stream, and a cooler holding gas stream; (vi) a separator to separate the cooler holding gas stream into a gaseous reduced-fuel stream and a liquid fuel stream; and (vii) a treatment unit to treat the gaseous reduced-fuel stream to further reduce the fuel percentage in the gaseous reduced-fuel stream and to provide a ventable stream. Optionally, the system of the present invention further comprises a reliquefaction system on the vessel to reliquefy the warmer cooling stream. 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. Optionally, the system of the present invention further provides wherein the fuel is ammonia. Optionally, the system of the present invention further provides wherein the treatment unit is one or more of the group comprising: a water bath, a catalytic reducer, a scrubber, an air diluter, a burner, or a combination of same. Optionally, the system of the present invention further comprises a post-separator holding tank for the gaseous reduced-fuel stream prior to the treatment unit. Optionally, the system of the present invention further comprises a compressor for the gaseous reduced-fuel stream prior to the treatment unit. Optionally, the system of the present invention further provides wherein the fuel tank of the vessel is a low temperature fuel tank. Optionally, the system of the present invention further provides wherein the temperature of the cooling stream is below 0°C, optionally in the range -5°C to -50°C, more optionally in the range -15°C to -25°C or -30°C. The present invention further provides a sea-going vessel having a reliquefaction system, an engine, and a system as defined herein for recovery of fuel from the engine. The present invention further provides a sea-going vessel as defined herein being a liquefied gas cargo carrier, and having an engine being a dual-fuel engine configured to work with a second fuel. The present invention also provides a vessel having a reliquefaction system, a fuel-fuelled engine, and a method as defined herein to recover fuel 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-LGIA engine produced by MAN Energy Solutions of 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 2 shows an example of a method and system of recovering a fuel and providing a ventable gas in a purging stream. In this example, a suitable fuel is ammonia, and the sea-going vessel is a liquefied gas cargo carrier having a cargo tank 116, and an ammonia-fuelled engine 112. The engine 112 may be one engine, or a series of engines, being supplied with fuel in arrangements known in the art. Figure 2 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 is a low-temperature tank, typically also under a relatively low pressure, and having 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 flow 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 2, 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 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 stream 140 from the catch tank 142 can flow / pass through one or more oilcoalescers 170 and one or more oil-absorbers 172 to reduce the oil content of the holding gas 140 prior to the lower temperature, (and the risk of oil-solidification). The holding gas 140 then enters a low-temperature heat exchanger 174, having for example a shell and coil arrangement. The coolant for the shell side of the low-temperature heat exchanger 174 can be supplied by a side-supply 160 from the cargo tank 116, which is already at a suitable low temperature. The action of the coolant is to cool the incoming holding gas 140 to provide a warmer cooling stream 171, and a cooler holding gas stream 177. The warmer cooling stream 171 from the heat exchanger comprises fuel, which can be then used as fuel, and / or reliqufied in a manner described herein if gaseous, using a reliquefaction system as described herein, optionally for return to the fuel tank as a liquid stream. If the warmer cooling stream 171 is a liquid stream, it can be then be used as fuel. The cooler holding gas stream 177 is a mixture of liquid, having a higher or richer amount of fuel relative to the holding gas, and vapour having a lower or leaner or reduced amount of fuel. The cooler holding gas stream 177 passes into a separator 175. The separator 175 may be or have any form or type or design known in the art, so as to provide a gaseous reduced-ammonia stream 176 at or near the top of the separator, and a liquid stream 173 at or near the bottom of the separator 175. The gaseous reduced-ammonia stream 176 has a lower, leaner or reduced amount or percentage of fuel therein, and thus a higher or richer amount of purge gas therein, e.g. a ‘fuel-poor’ stream. It is now easier and more efficient to treat such a stream as further described herein below, to wholly or substantially reduce the fuel content thereof prior to venting the remainder of the stream, (being now only purge gas). The liquid stream 173 has a lower, leaner or reduced amount or percentage of purge gas therein, and thus a higher or richer amount of fuel therein, e.g. a ‘fuel-rich’ stream. This stream 173 is now useable in the low-temperature heat exchanger 174 as additional coolant, prior to becoming part of the warmer cooling stream 177 from the heat exchanger 174, (and thus then reusable as fuel, or reliquifyable in a manner described herein, using a reliquefaction system as described herein, optionally for return to the fuel tank as a liquid stream). The gaseous reduced-ammonia stream 176 can be treated by a suitable unit 264 such as a water bath to further reduce the ammonia percentage in the gaseous reduced-fuel stream 176, and to provide a ventable stream 265 , i.e. a stream able to be vented to the ambient or atmosphere through a suitable vent 266, depending on local emission regulations. Optionally, the ventable stream is directly ventable to atmosphere or the ambient environment when allowed, or may undergo one or more further treatments until it is suitable for local emission regulations. The liquid fuel stream 173 provided by the separator 175 may be returned to the cargo tank 116, or be sent for use in the engine, or may be used as part of the cooling stream 160 required to pass into the low-temperature recovery heat exchanger 174, to provide cooling of the holding gas in a manner described above. That is, the liquid fuel stream may provide a suitable source or resource in combination with the cooling stream of fuel from the fuel tank 116, into the low-temperature recovery heat exchanger 174. Figure 2 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 2 also shows providing a ventable gas 265 directly from the holding gas stream 140, using a low-temperature coolant already available on the sea-going vessel. Figure 2 also shows the cargo tank 116 acting as both a general cargo tank for a liquefied ammonia 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 3 shows the option of including a holding tank 180 for the gaseous reduced-ammonia stream 176 prior to treatment by a treatment unit 264. Optionally, the gaseous reduced-ammonia stream 176 may pass through a compressor 178a The holding tank 180 provides a suitable location for holding the gaseous reduced-fuel stream, and any other gases or streams 182 on the sea-going vessel that require ammonia-reduction treatment. Such other gases or streams 182 may be at a lower pressure than the gaseous reduced-ammonia stream 176, and so could be compressed by a suitable compressor 178b prior to entering the holding tank 180, in order to reach the pressure in the holding tank 180. The holding tank 180 allows for flow organisation and timing into the one or more processes used in the treatment unit 264. The or each compressor(s) 178a, 178b allows a larger volume of gas to be stored in the holding tank 180. The treatment unit 264 may comprise one or more units, apparatus or devices able to provides one or more treatments of the gaseous reduced-fuel stream 176 in series or in parallel. In one embodiment, step (f) comprises passing the gaseous reduced-fuel stream 176 through a selective catalytic reducer. Figure 4 shows an arrangement of the low-temperature heat exchanger 174 to cool the incoming holding gas 140 to provide a warmer cooling stream 171, and a cooler holding gas stream 177, and the separator 175 to provide the gaseous reduced-ammonia stream 176 at or near the top of the separator, and a liquid stream 173 at or near the bottom of the separator 175. Figure 4 then shows using a suitable flow diverter 190 able to pass the gaseous reduced-fuel stream 176 either as a stream 179a to a water bath 184, or as a stream 17b to a catalytic reducer 186, or to both. The user can determine which treatment arrangement to use, and / or what percentage to deploy into which stream 179a, 179b, based on various factors and considerations, such as cost (i.e. CAPEX and OPEX), operational preferences, regulatory compliance, and / or the volume of the reduced-fuel stream that needs to be treated. In the arrangement shown in Figure 4, the holding tank 180 is shown in line with the catalytic reducer 186. Meanwhile, the water bath 184 will provide a treatment effluent 188 from its treatment of the gaseous reduced-fuel stream 176, and such effluent could be further provided from the water bath 184 to the catalytic reducer 186, for use in the reduction of the gaseous reduced-fuel stream 176 in the catalytic reducer. This arrangement may also remove the need to store the water bath effluent 188 from the sea-going vessel when in a port. In Figure 4, the water bath 184 can provide a first ventable stream 266a, and the catalytic reducer 186 can provide another ventable stream 266b. Figure 5 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. In the example of the present invention shown in Figure 5, 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. The holding gas steam 246 enters a low-temperature heat exchanger and separator unit 240, to provide a gaseous reduced-ammonia stream 238 that can undergo a further ammonia-content reduction through a suitable treatment unit 264 prior to a vent 266, whilst a gaseous stream 234 from the low-temperature heat exchanger can be treated by a reliquefaction unit 260, that is available for recovery of boil off gas (BOG) 232 from a cargo tank 230 in a manner known in the art. Ammonia recovery from the reliquefaction unit 216 can pass along 262 back into the ammonia fuel tank 212. A coolant stream 252 for the low-temperature heat exchanger and separator unit 240 can be supplied from the ammonia tank 212, either directly or as a bleed stream as shown from the engine supply pipeline 214. Liquid ammonia recovery from the low-temperature heat exchanger and separator unit 240 can be passed back into the ammonia fuel tank 212 via pipeline 251. Example 1 An engine was purged to provide a purging stream (e.g 122 in Figure 2) comprising a flow of about 50 kg ammonia and 80 kg of nitrogen. This purging stream was passed to holding tank (142) set at a pressure of 23 bar (absolute). From the holding tank, a holding stream (140) was supplied to a low-temperature recovery heat exchanger (174), being at least partly cooled by a cooling stream (160) of fuel provided from a fuel tank of a vessel. From the heat exchanger, the cooler holding gas stream (177) was provided at a pressure of approximately 21 bar (absolute) to a separator (175) Column 1 of Table 1 lists various different temperatures in the range of +0°C to -28°C for the cooling side of the low-temperature recovery heat exchanger. Column 2 of Table 1 lists remaining ammonia content of each gaseous reduced fuel stream (176) after the holding gas from the holding tank had passed through the low-temperature recovery heat separator through the separator. Column 3 of Table 1 lists the amount of ammonia recovered from the initial 50kg amount by the separator, (e.g. stream 173 in Figure 2), and able to be treated (using one or more of the processes or systems described herein) for return to storage or to the fuel system. Table 1 Catch tank at 23 bar abs, 4 bar dP 1. Temp °C 0 2. NH3 (kg) to abatement technique 16.0 3. NH3 (kg) recovered 36.8 69.7% -10 10.0 42.8 81.1% -15 7.9 44.9 85.1% -20 6.2 46.5 88.2% -25 4.9 47.9 90.8% -28 4.2 48.5 92.1% Example 1 confirms that with an increasingly lower temperature in the low-5 temperature recovery heat exchanger, an increasing amount of ammonia can be recovered from the purge stream. 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 fuel and a ventable stream from a purging stream purging an engine (112) of a sea-going vessel, the vessel having fuel in a low-temperature fuel tank configured to supply the engine, comprising at least the steps of:(a) purging the engine with a purging gas (120) to produce a fuel-containing purge gas stream (122);(b) passing the fuel-containing purge gas stream to one or more holding tanks (142) on the vessel to create a holding gas (140);(c) passing the holding gas into a low-temperature recovery heat exchanger (174);(d) passing a cooling stream of fuel (160) provided from the fuel tank (116) into the low-temperature recovery heat exchanger to cool the holding gas and to provide a warmer cooling stream (171), and a cooler holding gas stream (177);(e) passing the cooler holding gas stream into a separator (175) to provide a gaseous reduced-fuel stream (176) and a liquid fuel stream (173); and(f) treating the gaseous reduced-fuel stream to further reduce the fuel percentage in the gaseous reduced-fuel stream, and to provide a ventable stream (265).

2. A method as claimed in claim 1 wherein the method further comprises the step of:(g) passing the warmer cooling stream into a reliquefaction system or a refrigeration system provided on the vessel to provide a liquid fuel return stream, and returning the liquid fuel return stream into the fuel tank.

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

4. A method as claimed in any one of the preceding claims wherein the fuel is one or more of the group comprising: ammonia, liquefied petroleum gas, methane, ethane, propane, butane and dimethyl ether (DME); preferably ammonia.

5. A method as claimed in any one of the preceding claims wherein the vessel is a liquefied gas cargo carrier, and the cargo of the liquefied gas cargo carrier is selected from the group comprising: ammonia, liquefied petroleum gas, methane, ethane, propane, butane and dimethyl ether (DME); preferably 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 claims 4 to 6 wherein the fuel tank of the vessel is a low temperature fuel tank.

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

9. A method as claimed in any one of the preceding claims further comprising the step of passing the holding gas through one or more oil filters prior to step (c).

10. A method as claimed in any one of the preceding claims further comprising the step of recovering any liquid in the low temperature heat exchanger to the fuel tank.

11. A method as claimed in any one of the preceding claims wherein step (f) comprises passing the gaseous reduced-fuel stream through one or more of the group comprising: a water bath, a catalytic reducer, a scrubber, an air diluter, a burner, or a combination of same.

12. A method as claimed in claim 11, further comprising holding the gaseous reduced-fuel stream in a post-separator holding tank prior to step (f).

13. A method as claimed in any one of the preceding claims, further comprising compressing the gaseous reduced-fuel stream prior to step (f).

14. A method as claimed in any one of the preceding claims wherein step (f) comprises passing a portion of the gaseous reduced-fuel stream through a waterbath, and passing a portion of the gaseous reduced-fuel stream through a catalytic reducer.

15. A method as claimed in claim 14, wherein effluent from the water bath is used in the catalytic reduction of the gaseous reduced-fuel stream.

16. A method as claimed in any one of claims 11 to 15, wherein step (f) comprises passing the gaseous reduced-fuel stream through a selective catalytic reducer connected to the engine of the sea-going vessel.

17. A method as claimed in any one of the preceding claims further comprising the step of recovering the liquid fuel stream from the separator to the low temperature heat exchanger.

18. A method as claimed in any one of the preceding claims wherein the pressure of the holding gas is in the range 20-30 barg.

19. A method as claimed in any one of the preceding claims wherein the temperature of the cooling stream is below 0°C.

20. A method as claimed in claim 19 wherein the temperature of the cooling stream is in the range -5°C to -50°C, optionally in the range -15°C to -30°C.

21. A system for recovering fuel and a ventable stream from a purging stream purging an engine of a sea-going vessel, the vessel having fuel in a low-temperature fuel tank configured to supply the engine, comprising:(i) a source of purging gas configured to purge the engine with a purging gas to produce a fuel-containing purge gas stream:(ii) one or more holding tanks on the vessel configured to hold the fuelcontaining purge gas stream and to create a holding gas;(iii) a low-temperature recovery heat exchanger;(iv) a passageway to pass the holding gas into the low-temperature recovery heat exchanger;(v) a passageway to pass a cooling stream of fuel from the fuel tank into the low-temperature recovery heat exchanger to cool the holding gas and to provide a warmer cooling stream, and a cooler holding gas stream;(vi) a separator to separate the cooler holding gas stream into a gaseous reduced-fuel stream and a liquid fuel stream; and(vii) a treatment unit to treat the gaseous reduced-fuel stream to further reduce the fuel percentage in the gaseous reduced-fuel stream and to provide a ventable stream.22 A system as claimed in claim 21 further comprising a reliquefaction system on the vessel to reliquefy the warmer cooling stream.

23. A system as claimed in claim 22 wherein 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 (LPG), propylene, ethylene and ammonia.

24. A system as claimed in claim 22 or claim 23 wherein the reliquefaction system is a cargo reliquefaction system for recovering boil-off gas of the cargo.

25. A system as claimed in any one of claims 21 to 24 wherein the fuel is ammonia.

26. A system as claimed in any one of claims 21 to 25 wherein the treatment unit is one or more of the group comprising: a water bath, a catalytic reducer, a a scrubber, an air diluter, or a combination of same.

27. A system as claimed in claim 26 further comprising a post-separator holding tank for the gaseous reduced-fuel stream prior to the treatment unit.

28. A system as claimed in any one of claims 21 to 27 wherein the fuel tank of the vessel is a low temperature fuel tank.

29. A system as claimed in any one of claims 21 to 28 wherein the temperature of the cooling stream is below 0°C, optionally in the range -5°C to -50°C, more optionally in the range -15°C to -30°C.

30. A sea-going vessel having a reliquefaction system, an engine, and a system as defined in any one of claims 21 to 29 for recovery of fuel from the engine.

31. A sea-going vessel having a reliquefaction system, an engine, and a method 5 as defined in any one of claims 1 to 20 to recover fuel from the engine.

32. A vessel as claimed in claim 30 or claim 31 being a liquefied gas cargo carrier, and having an engine being a dual-fuel engine configured to work with a second fuel.10

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