WO2015063453A2

WO2015063453A2 - Method and system for the re-liquefaction of boil-off gas

Info

Publication number: WO2015063453A2
Application number: PCT/GB2014/053090
Authority: WO
Inventors: Nicola CASTELLUCCI
Original assignee: Highview Enterprises Limited
Priority date: 2013-10-28
Filing date: 2014-10-15
Publication date: 2015-05-07
Also published as: GB201318996D0; EP3063486B1; CN105683690B; CN105683690A; PT3063486T; JP6591410B2; DK3063486T3; JP2016535211A; EP3063486A2; GB2519594A; ES2819212T3; PL3063486T3; WO2015063453A3

Abstract

A method for liquefying boil-off gas comprises storing a liquefied hydrocarbon gas in a store; processing streams of gaseous cryogenic fluid and liquefied hydrocarbon gas by transferring heat, such that the liquefied hydrocarbon gas becomes gaseous and the gaseous cryogenic fluid becomes liquefied;storing the liquefied cryogenic fluid in a store;processing streams of gaseous boil-off gas and liquefied cryogenic fluid by transferring heat, such that the liquefied cryogenic fluid becomes gaseous and the gaseous boil-off gas becomes liquefied; and storing the liquefied boil-off gas in the store. The method further comprises controlling the flow rate of the gaseous cryogenic fluid based in part on the flow rate of the liquefied hydrocarbon gas and independently controlling the flow rate of the liquefied cryogenic fluid based in part on the flow rate of the gaseous boil-off gas. A corresponding system is provided.

Description

Method and System for the Re-Liquefaction of Boil-off Gas Field of the invention

The present invention relates to a method and system for re-liquefying boil-off gas by processing a stream of hydrocarbon gas, a stream of cryogenic fluid, and a stream of boil-off gas. More particularly, the present invention relates to controlling the flow rate of the stream of cryogenic fluid based in part on the flow rates of the streams of hydrocarbon gas and boil-off gas.

Background of the invention

Natural gas is a key source of energy for the world economy; it is estimated that natural gas supplies approximately one-fifth of global energy needs. This compares to one-third and one- quarter for oil and coal respectively. As is generally the case with bulk energy commodities, natural gas reserves do not lie near the major areas of demand, and so natural gas must be transported and traded internationally. Approximately 30% of natural gas produced globally is traded on the world market.

The two principal methods for transporting natural gas are: a) transporting in gaseous form in pipelines; and b) transporting in liquid form as liquefied natural gas (LNG) in transport vessels.

To transport natural gas in liquid form as LNG, the gas must be liquefied (i.e. changed from a gaseous state to a liquid state). The liquefaction of LNG is an energy intensive process and so is more economical for long distance transport; in particular across oceans. As a result, LNG accounts for nearly three-quarters of long-distance natural gas trade. Due to the energy required for its liquefaction, LNG contains a large quantity of embodied cold energy which is released when it is re-gasified (i.e. changed from its liquid state following liquefaction back into its gaseous state).

The use of LNG in recent years has risen significantly as a share of both gas production and trade. Global LNG trade has more than doubled since 2000, while pipeline trade has risen by only around one-third.

In the Atlantic natural gas market, pipeline trade and local gas production have a dominant market share, which tends to favour inter-basin trading; particularly in the UK where LNG import terminals have seen a downturn in utilisation over the past few years, with cargoes being diverted to the Asia-Pacific in search of higher prices. In such a competitive market, the flexibility and efficiency of LNG import terminals is particularly important. The owners of LNG infrastructure such as LNG import terminals therefore seek further improvements in handling, storage and re- gasification of LNG.

LNG import terminals typically receive LNG from a transport vessel, such as a specially designed cargo ship, and pump it into large capacity low-pressure storage tanks, where it is stored at cryogenic temperatures (around -163 °C). When market conditions are favourable, LNG is pumped to high pressure, warmed and vaporised before being exported on the gas network. The export rate, or nomination, is highly dependent on gas price.

In recent years, the UK LNG market has experienced volatile gas prices, leading to fluctuating export and significant periods of zero export nomination from LNG terminals. Figure 6 shows an example profile of a year's send-out from an LNG terminal. These conditions require a liquefaction plant to be as flexible and efficient as possible to enable operators to have maximum control over when and how much LNG is exported, whilst maximising storage capacity and longevity.

In any thermal process, efficiency losses occur when heat is allowed to flow in to or out of the process. Due to the low temperatures involved in cryogenic systems, a significant source of uncontrolled heat is the ambient environment. This heat may enter the system through pipe and vessel walls. In an LNG infrastructure the ingress of heat results in the loss of LNG through evaporation. This is commonly known in the industry as boil-off and the resulting vapour phase as boil-off gas (BOG).

It is widely understood that over long periods, a significant proportion of LNG may be lost through boil-off. In a well-insulated LNG tank, a typical boil-off rate may be 0.05% of the volume per day. However, this rate may increase up to 3 times or more depending on the design and operational requirements of the plant. The boil-off rate may be even higher during transients such as unloading of an LNG cargo.

Furthermore, LNG is a multi-component fluid (typically composed of methane, ethane, nitrogen, propane and butane) and it is widely understood that during the storage and handling of such multi-component cryogenic fluids, boil-off may result in a change in their component concentration. This is the result of the different volatilities of the component fluids. Heat ingress will cause the components to evaporate at different rates. The more volatile components (with lower saturation temperatures for a fixed pressure) will tend to evaporate first and the liquid phase will therefore become more concentrated in the less volatile components. This represents an additional problem as strict regional standards for natural gas composition must be respected. Over time, evaporation leads to a costly degradation of the LNG stock. The ratio of the calorific value and the density of the gas (the Wobbe index) must subsequently be controlled by the reinjection of LNG components, typically propane and nitrogen.

It is therefore of critical importance to carefully manage LNG stocks to minimise losses through boil-off.

The higher the rate of heat flow into the process the faster the rate of boil-off. In an LNG infrastructure, the rate of heat flow is minimised primarily by insulating the infrastructure from the surrounding ambient environment. For example, a typical LNG tank is well insulated in order to minimise the ingress of heat. Although particular to the infrastructure design and operation, further limitation of boil-off may typically be achieved through management of tank levels, optimised timing of deliveries, and cooling of key systems.

For example, during unloading of LNG to an import terminal, the transfer of heat from warm pipework to the incoming LNG causes the boil-off rate to increase. This may result in a peak in the rate of boil-off. Often it is preferable to maintain the pipework at cryogenic temperatures by active cooling. This allows the plant to remain in a state of readiness, improving reactivity. This may be achieved most effectively by continuously running LNG through the pipelines. This represents a trade-off, inducing a higher continuous boil-off rate in order to maintain the pipework at operational temperature.

It is widely appreciated that boil-off cannot be completely eliminated. However, the loss of LNG stock through boil-off may be eliminated by re-liquefying the boil-off gas and returning it to storage in its liquid form. The full volume of LNG is thus retained and the degradation of the LNG composition is avoided, thus increasing the longevity of the stock. Re-liquefaction is achieved by compressing, cooling and in some cases expanding the boil-off gas. Typically, cooling is achieved using closed-loop refrigeration cycles with a refrigerant fluid. Sometimes the boil-off gas may be employed as a refrigerant fluid by returning a portion of cooled or re-liquefied boil-off gas to the system to perform cooling. The process of re-liquefaction is energy intensive and represents a high operating cost.

Where re-liquefaction is too costly, all or a portion of the boil-off gas may be utilised to offset the operating costs of the plant. Examples include extracting useful heat or work from combustion. The benefits of this solution vary according to market conditions as the boil-off gas used in this way is diverted from the gas market. In some cases there may not be sufficient energy requirement in the plant and it is often more cost effective to import energy from external sources. Boil-off gas may alternatively be sent out on the local or regional gas network, but compressing the gaseous boil-off gas to the required pressure for the network is costly. To reduce energy requirements the boil-off gas is often condensed into a stream of supercooled LNG. The resulting liquid may be pumped to higher pressure and gasified to achieve the required network pressure. Alternatively, the boil-off gas may be re-liquefied in heat exchange with a stream of LNG before being mixed in its liquid phase. In any case, since boil-off gas is richer in the more volatile components of LNG, mixing with LNG allows the criteria for gas composition to be respected. However, during this process, up to two units or more of re-gasified LNG must be added to one unit of boil-off gas. This often results in a minimum rate of continuous export that is considerably greater than the actual boil-off rate. This minimum send-out rate limits the flexibility of the plant to respond to market conditions. Moreover, since the export of LNG is required for this process, continuous operation of the re-gasification plant is necessary.

The advantages of boil-off gas re-liquefaction are evident. Re-liquefaction represents a means of addressing both the loss of LNG over time through boil-off and the degradation of the LNG stock. The operator is afforded maximum control over when and how much gas is exported; crucially, the operator is not required to export gas during unfavourable market conditions.

However, the operating costs of re-liquefaction processes are usually prohibitive, especially in large infrastructures with large amounts of pipework, where high levels of boil-off occur, and where active cooling is employed. These operating costs arise from the work required by the process, which is generally provided by electric motors.

A re-liquefaction process requires the input of work to compress the working fluid. The fluid is then cooled by a cold source. Those skilled in the art will recognise that the quantity of work required to achieve the required cooling is dependent on the temperature of the cold source. Where the cold source is at ambient temperature, a greater quantity of work is required. Where the cold source is below ambient temperature, for example at cryogenic temperature, the quantity of work required is greatly reduced.

One source of cold in an LNG import terminal is the re-gasification of LNG, which is heated from approximately -163°C to near-ambient temperature. The cold recovered from this process is often dissipated as waste. However, if this cold is recovered and recycled in a liquefaction process, the electrical consumption of the process may be reduced by as much as two thirds. This approach has been adopted in the design of nitrogen liquefiers and air separation plants integrated into LNG infrastructure, a number of which are in operation in Japan and Korea. US 4329842 describes a system for utilising cold energy from regasification of LNG at an LNG vaporising plant. LNG is taken from an LNG source ship and passed through to a pipeline via a liquid air generating plant where it is used generate liquid air for subsequent use in a power generation system.

However, it has been established that the re-liquefaction of boil-off gas is of primary importance during unfavourable market conditions, at a time when the cold from re-gasification of LNG is not available. This "anti-phase" between the requirement for and the availability of cold has hitherto prevented the cold from LNG re-gasification being used to re-liquefy boil-off gas during such periods.

US 3400547 discloses a process for utilising a cryogenic fluid to facilitate generation and transport of LNG. Cold energy from evaporation of LNG at a market site is used to liquefy nitrogen, which is transported to the field. Here, cold energy from the liquefied nitrogen is used to liquefy natural gas to form LNG, which is transported back to the market site.

US2007/0186563 discloses a method of cold recovery in a cold compressed natural gas cycle. Cold energy from cold compressed natural gas in a cavern is used to liquefy air for storage, with the resulting natural gas being distributed via pipeline. Natural gas may be drawn from the pipeline, cooled using cold energy form the liquefied air, and stored in the cavern.

Neither of these documents offers a solution to the problems described above of how to deal efficiently with the problem of boil-off. Accordingly, an improved method and system is required for liquefying boil-off gas which overcomes the above-mentioned problems.

Summary of the invention

Accordingly, in a first aspect, the present invention provides a method for liquefying boil-off gas, comprising:

storing a liquefied hydrocarbon gas in a liquefied hydrocarbon gas store;

processing a stream of gaseous cryogenic fluid and a stream of liquefied hydrocarbon gas from the liquefied hydrocarbon gas store, such that:

a) the stream of liquefied hydrocarbon gas undergoes a phase change from a liquefied hydrocarbon gas to a gaseous hydrocarbon gas; and

b) the stream of gaseous cryogenic fluid undergoes a phase change from a gaseous cryogenic fluid to a liquefied cryogenic fluid; wherein the step of processing comprises transferring heat from the stream of gaseous cryogenic fluid to the stream of liquefied hydrocarbon gas from the liquefied hydrocarbon gas store; storing the liquefied cryogenic fluid in a liquefied cryogenic fluid store;

processing a stream of gaseous boil-off gas and a stream of liquefied cryogenic fluid from the liquefied cryogenic fluid store such that:

a) the stream of liquefied cryogenic fluid undergoes a phase change from a liquefied cryogenic fluid to a gaseous cryogenic fluid; and

b) the stream of gaseous boil-off gas undergoes a phase change from a gaseous boil-off gas to a liquefied boil-off gas;

wherein the step of processing comprises transferring heat from the stream of gaseous boil-off gas to the stream of liquefied cryogenic fluid from the liquefied cryogenic fluid store;

storing the liquefied boil-off gas in the liquefied hydrocarbon gas store;

controlling the flow rate of the stream of gaseous cryogenic fluid based at least in part on the flow rate of the stream of liquefied hydrocarbon gas from the liquefied hydrocarbon gas store; and

independently controlling the flow rate of the stream of liquefied cryogenic fluid from the liquefied cryogenic fluid store based at least in part on the flow rate of the stream of gaseous boil- off gas.

By performing the steps above, an improved method of re-liquefying boil-off gas is achieved through effective recovery, storage and recycling at a later time of the cold energy released during re-gasification of a hydrocarbon gas. In some circumstances, the energy required to re-liquefy boil-off gas using the method of the invention may be more than halved compared with conventional methods. The energy requirements for the method of the invention are low enough to be implemented in existing hydrocarbon gas infrastructure. Thus, the method provides a cost- effective technique which improves flexibility of managing the export of hydrocarbon gas according to market conditions; increases the longevity of storage; and effectively increases the storage volume of the hydrocarbon gas tanks by ensuring hydrocarbon gas used in continuous cooling is not lost. It is particularly advantageous in that it reduces the work required for the re-liquefaction of boil-off gas by the recycling of cold available on site that would otherwise be unavailable when required.

A particular advantage of the present invention is that cold from the re-gasification of hydrocarbon gas may be recovered, stored and utilised in a process for the re-liquefaction of boil- off gas independently of the rate and time of cold recovery. In particular, by storing a liquefied cryogenic fluid in a fluid store, and by controlling the flow rate of the cryogenic fluid into and out of the store, it is possible to make use of cold recovered from regasification of the liquefied hydrocarbon gas whilst that process is taking place; store the recovered cold in the fluid store; and utilise it when required to re-liquefy boil-off gas. The steps of storing and controlling the cryogenic fluid enable energy to be transferred between two processes even if those processes are not taking place at the same time.

The present invention is particularly useful at LNG import terminals and any other LNG storage infrastructure with a regasification plant, where the cold from re-gasification of LNG may be recovered and utilised for the re-liquefaction of boil-off gas. However, it would also be applicable to boil-off from other high volume cryogenic storage systems where the cold from regasification is periodically available.

It should be noted that, for convenience, the description and claims refer to the cryogenic fluid, boil-off gas and hydrocarbon gas in their gaseous and liquefied forms. It should be understood that in each case, the same fluid is being referred to albeit in a different phase. For instance, the invention mentions a liquefied cryogenic fluid. It will be understood that this is the liquefied state of the stream of gaseous cryogenic fluid which is also mentioned.

It should also be noted that, for consistency of nomenclature, the cryogenic fluid is described as such in both its gaseous and liquefied forms irrespective of the temperature of the fluid. Hence, in certain circumstances the gaseous cryogenic fluid may be at near-ambient or above ambient temperatures. Regardless, it is referred to in this application as a cryogenic fluid because it is utilised to transfer heat to and from fluids at cryogenic temperatures.

Finally, whilst it is appreciated that 'cold' is merely the absence of energy, rather than a form of energy itself, it is convenient to use the expression 'cold energy' in a discussion of energy transfer in a cryogenic energy system because it is typically cold temperatures which are sought to be preserved and ingress of heat energy which is sought to be excluded. The skilled reader will appreciate that in this sense, 'cold energy' is a convenient fiction for describing this technology and is analogous to the transfer and preservation of heat energy in non-cryogenic systems.

The method may further comprise the step of processing the stream of gaseous boil-off gas and the stream of liquefied hydrocarbon gas from the liquefied hydrocarbon gas store such that:

a) the stream of liquefied hydrocarbon gas undergoes a phase change from a liquefied hydrocarbon gas to a gaseous hydrocarbon gas; and b) the stream of gaseous boil-off gas undergoes a phase change from a gaseous boil-off gas to a liquefied boil-off gas;

wherein the step of processing comprises transferring heat from the stream of gaseous boil-off gas to the stream of liquefied hydrocarbon gas from the liquefied hydrocarbon gas store.

This method is advantageous because it permits the boil-off gas to be re-liquefied whilst regasification of the liquefied hydrocarbon gas is taking place, as well as at a later time using the cold stored in the cryogenic fluid. This further improves the efficiency of the process because cold energy from regasification can be used to cool boil-off gas directly, whereas cooling using the cryogenic fluid may be reserved for when regasification is not taking place.

In the case mentioned above, the steps of: a) transferring heat from the stream of gaseous cryogenic fluid to the stream of liquefied hydrocarbon gas from the liquefied hydrocarbon gas store; and b) transferring heat from the stream of gaseous boil-off gas to the stream of liquefied hydrocarbon gas from the liquefied hydrocarbon gas store; may either be concurrent or not concurrent.

When the steps are concurrent, the cold energy from regasification is used to re-liquefy boil-off gas and cool and liquefy the cryogenic fluid for later use. This may be particularly preferable if there is a plentiful supply of cryogenic fluid; stocks of liquefied cryogenic fluid in the store are low; and/or a long delay is expected until the next regasification of hydrocarbon gas. When the steps are not concurrent, the cold energy from regasification may be used to re-liquefy boil-off gas without cooling and liquefying cryogenic fluid (which may be particularly preferable when there is a sparse supply of cryogenic fluid; stocks of liquefied cryogenic fluid in the store are high; and/or a short delay is expected until the next regasification of hydrocarbon gas) or cool and liquefy the cryogenic fluid without re-liquefying boil-off gas (which may be particularly preferable when there is little or no boil-off gas to be re-liquefied, or the cryogenic fluid store is empty).

The step of processing the stream of gaseous cryogenic fluid and the stream of liquefied hydrocarbon gas may further comprise one or both of the steps of: expanding the stream of gaseous cryogenic fluid after heat transfer; and compressing the stream of gaseous cryogenic fluid prior to heat transfer. The stream of gaseous cryogenic fluid may be compressed to a supercritical pressure.

In certain circumstances, the transfer of heat itself is sufficient to effect the change of phase from liquid to gas and vice versa. In these circumstances one fluid will enter a heat exchange (for example) in the liquid phase and exit in the gaseous phase whilst the other will enter the heat exchange in the gaseous phase and exit in the liquid phase. However, in practice this is not always possible or convenient, and the process is made more efficient by one or both of compressing and expanding one or more of the fluids before and after heat transfer. In the present case it has been found advantageous to expand the gaseous cryogenic fluid after heat transfer to achieve liquefaction and compress the gaseous cryogenic fluid before heat transfer.

The method may further comprise the steps of passing the stream of liquefied hydrocarbon gas through first and second branches. In that case, the step of transferring heat from the stream of gaseous cryogenic fluid to the stream of liquefied hydrocarbon gas from the liquefied hydrocarbon gas store may further comprise:

transferring heat to a stream of liquefied hydrocarbon gas in the first branch from the stream of gaseous cryogenic fluid prior to compression; and

transferring heat to a stream of liquefied hydrocarbon gas in the second branch from the stream of gaseous cryogenic fluid after compression.

Preferably, the method further comprises combining the streams of gaseous hydrocarbon gas in the first and second branches.

Passing the stream through first and second branches enables the cold energy transferred from the liquefied hydrocarbon gas to be used in more than one place. In particular, it is advantageous for the gaseous cryogenic gas to undergo initial cooling, prior to compression for example, and then to undergo subsequent cooling to liquefy the cryogenic gas. By providing first and second streams of liquefied hydrocarbon gas, both stages of cooling can be achieved by the cold energy from the regasification process.

It will be understood that hydrocarbon gas finds many uses in commercial and residential properties, as well as in industry and the plants themselves. Preferably, the method further comprises the step of delivering the stream of gaseous hydrocarbon gas to a recipient such as one or more of: a hydrocarbon pipe network; a power station; and a consumer of gaseous hydrocarbon gas.

Preferably, the method further comprises the step of collecting the stream of gaseous boil- off gas, such as by collecting the boil-off gas from the liquefied hydrocarbon gas store and/or collecting the boil-off gas from a store, conduit, or collection point coupled to the liquefied hydrocarbon gas store. Boil-off can occur wherever liquefied hydrocarbon gas is present and at risk of being warmed through insufficient insulation. The skilled person is familiar with methods for collecting this boil-off from all over an infrastructure, wherever it occurs - even very far from the tank - and thus efficiencies can be increased.

The step of transferring heat from the stream of gaseous cryogenic fluid to the stream of liquefied hydrocarbon gas from the hydrocarbon gas store may be direct, or it may comprise transferring heat from the stream of gaseous cryogenic fluid to a heat transfer fluid in a closed-loop refrigeration circuit and cooling the gaseous cryogenic fluid to a temperature below the saturation temperature of the liquefied hydrocarbon gas; and transferring heat from the heat transfer fluid in the closed-loop refrigeration circuit to the stream of liquefied hydrocarbon gas.

Heat transfer may take place directly; that is, between two streams of fluid in a single heat exchange, or indirectly via one or more refrigeration circuits (or equivalent), wherein cold from a source stream is passed to one or more intermediate streams of heat transfer fluid before reaching its destination stream. In the preferred example, cold from the stream of liquefied hydrocarbon gas (i.e. the source stream) is passed to a closed-loop refrigeration circuit before reaching the stream of gaseous cryogenic fluid (i.e. the destination stream). The closed-loop refrigeration circuit may also involve expanding and compressing the heat transfer fluid to obtain the required temperatures.

In cases where heat from the stream of gaseous boil-off gas is transferred to the stream of liquefied hydrocarbon gas from the liquefied hydrocarbon gas store, the step of transferring heat from the stream of gaseous boil-off gas to the stream of liquefied hydrocarbon gas from the liquefied hydrocarbon gas store may further comprise:

transferring heat from the stream of gaseous boil-off gas to the heat transfer fluid in the closed-loop refrigeration circuit; and

transferring heat from the heat transfer fluid in the closed-loop refrigeration circuit to the stream of liquefied hydrocarbon gas.

It will be appreciated that the destination stream for the cold energy which passes from the source stream through the one or more intermediate streams may be more than one stream. In the preferred example, cold energy is transferred not only to the stream of gaseous cryogenic gas, but also to the stream of gaseous boil-off gas.

Preferably, the method further comprises processing a stream of ambient air to form the stream of gaseous cryogenic fluid. This may involve, for example, filtering the stream of ambient air to remove moisture, carbon dioxide and/or hydrocarbons; and/or compressing the stream of ambient air. Air is particularly advantageous due to its abundance. This permits a readily available supply of gaseous cryogenic fluid for use on demand.

Preferably, the method further comprises passing the stream of liquefied cryogenic fluid through a separator prior to it entering the liquefied cryogenic fluid tank to separate any residual vapour phase from the stream of liquefied cryogenic fluid, and returning the residual vapour phase to the stream of gaseous cryogenic fluid.

It will be appreciated that cryogenic fluid may suffer boil-off within the infrastructure itself, in particular before the liquefied cryogenic fluid enters the store. Moreover, the liquefaction of cryogenic fluid may not be 100% efficient, and there may be cryogenic fluid in the vapour or gas phase even after the stream has been processed. In these circumstances, separating the vapour or gas phase and returning it to the gaseous stream of cryogenic fluid is particularly advantageous because the efficiency of the liquefaction process is improved.

Preferably, the method further comprises pumping the stream of liquefied cryogenic fluid from the liquefied cryogenic fluid store to increase its pressure prior to the step of transferring heat from the stream of gaseous boil-off gas to the stream of liquefied cryogenic fluid from the liquefied cryogenic fluid store.

Preferably, the step of transferring heat from the stream of gaseous boil-off gas to the stream of liquefied cryogenic fluid from the liquefied cryogenic fluid store results in a second stream of gaseous cryogenic fluid. In that case, the method may further comprise the step of expanding the second stream of gaseous cryogenic fluid to extract work from the stream.

The step of expanding the second stream of gaseous cryogenic fluid to extract work from the second stream may be performed in a single-stage expansion device, a two-stage expansion device, or a multi-stage expansion device.

Preferably the method further comprises super-heating the second stream of gaseous cryogenic fluid prior to one or more stages of expansion. The heat source for super-heating the cryogenic fluid may be ambient air. It may otherwise be any heat source from a co-located process with a temperature of up to 500°C, for instance.

Preferably the method further comprises the step of converting the work extracted from the second stream into electricity. By extracting work from the gaseous cryogenic fluid used to re-liquefy the boil-off gas, the work required by the process (such as the work done in compressing the gaseous cryogenic fluid and/or pumping the liquefied cryogenic fluid) may be offset. Steps of increasing the pressure of the liquefied cryogenic fluid, and expanding and superheating the cryogenic fluid increase the efficiency by which work may be extracted from the stream. This work may be converted to electricity using an electric generator.

In a second aspect, the present invention provides a system for liquefying boil-off gas, comprising:

a first store for storing liquefied hydrocarbon gas;

a first arrangement of conduits coupled to the first store and to a hydrocarbon gas network for delivering hydrocarbon gas to a recipient;

a second arrangement of conduits coupled to a source of boil-off gas and to the first store for delivering liquefied boil-off gas to the first store;

a second store for storing a liquefied cryogenic fluid;

a third arrangement of conduits coupled to a source of gaseous cryogenic fluid and the second store for delivering liquefied cryogenic fluid to the second store;

a fourth arrangement of conduits coupled to the second store for delivering cryogenic fluid from the second store; wherein:

the first and third arrangements of conduits are arranged such that heat is transferred from a stream of gaseous cryogenic fluid passing through the third arrangement of conduits to a stream of liquefied hydrocarbon gas passing through the first arrangement of conduits;

the second and fourth arrangements of conduits are arranged such that heat is transferred from a stream of gaseous boil-off gas passing through the second arrangement of conduits to a stream of liquefied cryogenic fluid passing through the fourth arrangement of conduits; and

a controller configured to:

a) control the flow rate of the stream of gaseous cryogenic fluid passing through the third arrangement of conduits based at least in part on the flow rate of the stream of liquefied hydrocarbon gas passing through the first arrangement of conduits; and b) independently control the flow rate of the stream of liquefied cryogenic fluid passing through the fourth arrangement of conduits based at least in part on the flow rate of the stream of gaseous boil-off gas passing through the second arrangement of conduits.

Most of the advantages associated with the second aspect of the present invention have already been described above in connection with the first aspect. Hence, for conciseness, they are not repeated here. The first and second arrangements of conduits may be arranged such that heat is transferred from the stream of gaseous boil-off gas passing through the second arrangement of conduits to the stream of liquefied hydrocarbon gas passing through the first arrangement of conduits.

The third arrangement of conduits may comprise a compressor for compressing the stream of gaseous cryogenic fluid. In that case, the first arrangement of conduits may comprise a first branch and a second branch. The first branch is preferably arranged such that heat is transferred to a stream of liquefied hydrocarbon gas passing through the first branch from the stream of gaseous cryogenic fluid passing through the third arrangement of conduits at a first heat exchange region upstream of the compressor. The second branch is preferably arranged such that heat is transferred to a stream of liquefied hydrocarbon gas passing through the second branch from a stream of gaseous cryogenic fluid passing through the third arrangement of conduits at a second heat exchange region downstream of the compressor.

The first and second branches may bifurcate from a single conduit upstream of the first and second heat exchange regions, and recombine to a single conduit downstream of the first and second heat exchange regions.

The source of boil-off gas may be the first store, and/or a store, conduit, or collection point coupled to the first store.

The first and third arrangements of conduits may be arranged such that heat is transferred between the first and third arrangements of conduits via a closed-loop refrigeration circuit comprising a heat transfer fluid passing through a fifth arrangement of conduits. In that case, the fifth and third arrangements of conduits may be arranged such that heat is transferred from the stream of gaseous cryogenic fluid passing through the third arrangement of conduits to the heat transfer fluid passing through the fifth arrangement of conduits. The fifth and first arrangements of conduits may be arranged such that heat is transferred from the heat transfer fluid passing through the fifth arrangement of conduits to the stream of liquefied hydrocarbon gas passing through the first arrangement of conduits.

If the first and second arrangements of conduits are arranged such that heat is transferred from the stream of gaseous boil-off gas passing through the second arrangement of conduits to the stream of liquefied hydrocarbon gas passing through the first arrangement of conduits, then the first and second arrangements of conduits may also be arranged such that heat is transferred between the first and second arrangements of conduits via the closed-loop refrigeration circuit. In that case, the fifth and second arrangements of conduits may be arranged such that heat is transferred from the stream of gaseous boil-off gas passing through the second arrangement of conduits to the heat transfer fluid passing through the fifth arrangement of conduits.

If the first arrangement of conduits comprises first and second branches, then the second branch may be arranged such that heat is transferred from the heat transfer fluid passing through the fifth arrangement of conduits to the stream of liquefied hydrocarbon gas passing through the second branch.

Preferably, the stream of gaseous cryogenic fluid air, and the third arrangement of conduits further comprises one or both of: a filtration system for removing moisture, carbon dioxide and/or hydrocarbons from a stream of ambient air; and a compressor for compressing a stream of ambient air.

The third arrangement of conduits may further comprise a separator upstream of the second store for extracting any residual vapour phase from the stream of liquefied cryogenic fluid passing through the third arrangement of conduits prior to entering the second store, and a return conduit arranged to direct the residual vapour phase extracted from the stream of liquefied cryogenic fluid to the stream of gaseous cryogenic fluid passing through the third arrangement of conduits.

Preferably, the second and fourth arrangements of conduits are arranged such that heat is transferred between the second and fourth arrangements of conduits at a third heat exchange region and the fourth arrangement of conduits further comprises a pump upstream of the third heat exchange region for pumping the stream of liquefied cryogenic fluid passing through the fourth arrangement of conduits prior to it passing through the third heat exchange region.

Preferably, the third heat exchange region is configured such that heat is transferred from the stream of gaseous boil-off gas passing through the second arrangement of conduits to the stream of liquefied cryogenic fluid passing through the fourth arrangement of conduits to produce a second stream of gaseous cryogenic fluid. In that case, the fourth arrangement of conduits further comprises an expansion device for expanding the second stream of gaseous cryogenic fluid and extracting work from the second stream of cryogenic fluid.

The expansion device may be a single-stage expansion device, a two-stage expansion device, or a multi-stage expansion device. Preferably, the fourth arrangement of conduits is coupled to one or more super-heaters, wherein each super-heater is either upstream of the first stage of the expansion device or between stages of the expansion device. In one example, if the expansion device has three expansion stages, and the fluid passing through is superheated before passing through each stage, then the system will comprise a first superheater upstream of the first stage, a second superheater between the first and second stages, and a third superheater between the second and third stages. In this context, the terms 'upstream' and 'between' do not preclude the possibility of there being other components (valves, an suchlike) between a superheater and a respective stage. It will be appreciated that not every stage need have a corresponding superheater. For a given arrangement in an expansion device, any number of superheaters may be provided in any arrangement appropriate for the circumstances.

In a preferred embodiment, the first, second, third and fourth arrangements of conduits are arranged such that heat is transferred between the first and third arrangements of conduits, between the second and fourth arrangements of conduits, at a single heat exchange region.

It will be appreciated that further efficiencies, both in terms of heat transfer as well as space, may be achieved by providing a single heat exchange region to for effecting more than one transfer of heat between two different streams. The heat exchange region may be provided by a single heat exchange (i.e. such that heat transfer is effected directly), or by a plurality of heat exchangers (i.e. such that heat transfer is effected via one or more intermediate streams such as the aforementioned closed-loop refrigeration circuit.

More preferably, the first, second, third and fourth arrangements of conduits are arranged such that heat is transferred between the first and second arrangements of conduits at the single heat exchange region.

The closed-loop refrigeration circuit mentioned above may operate using one of a single- phase Brayton cycle and a dual-phase Rankine cycle.

The heat transfer fluid may be any fluid with the appropriate thermo-dynamic properties with respect to the saturation temperatures of the hydrocarbon gas and the cryogenic fluid. For example, nitrogen or propane may be used, both of which are typically available at a hydrocarbon gas terminal. The cryogenic fluid mentioned above may be one of nitrogen or air, preferably ambient air. Nitrogen is typically available at a hydrocarbon gas terminal and requires minimal processing before it can be used, whereas air is abundant.

Finally, it should be noted that the liquefied hydrocarbon gas mentioned herein is preferably Liquefied Natural Gas (LNG). LNG is the predominant kind of hydrocarbon gas in current supply, and therefore the present invention finds particular utility with LNG. However, the present invention may be implemented with any hydrocarbon gas wherein the re-liquefaction of boil-off in any application where a hydrocarbon which is normally in its gaseous phase under ambient conditions is stored as a cryogenic liquid in large quantities and then re-gasified for use.

Brief description of the drawings

Preferred embodiments of the invention will now be described with reference to the accompanying drawings, in which: figure 1 is a diagram of a system according to a first embodiment of the invention; figure 2 is a diagram of a system according to a second embodiment of the invention; figure 3 is a diagram of a system according to a third embodiment of the invention; figure 4 is a diagram of a system according to a fourth embodiment of the invention; figure 5 is a diagram of a system according to a fifth embodiment of the invention; and figure 6 is a graph depicting an example of the gas send-out of an LNG terminal over a year.

Detailed description of the invention

The present inventors have previously disclosed, in patent application number WO200709665, a cryogenic energy storage system which stores energy using a cryogenic fluid. The present inventors have also described, in UK patent application number 1305640.3, an efficient method of cooling within an air liquefaction processes using cold recovery from adjacent LNG re-gasification process. Both of these disclosures are helpful, but not necessary, for putting the present invention into practice. A first embodiment of the present invention uses a cryogenic fluid, such as liquid air or liquid nitrogen, to store the cold from the re-gasification of LNG. A system diagram of the first embodiment is presented in figure 1.

During re-gasification, the LNG is pumped to high pressure and split into two streams, whereby the first stream is warmed and vaporised in heat exchange with the cryogenic fluid in its gaseous phase; and the second stream is warmed and vaporised in heat exchange with a refrigerant, typically nitrogen, in a closed-loop refrigeration cycle.

The two, now gaseous, streams are then merged into a single stream of gaseous natural gas for export. The re-gasified natural gas is sent, as known in the art, to a recipient, which may form part of the LNG infrastructure or be an external infrastructure or customer. Examples include, but are not limited to: a gas sendout station, a pipe network, a power station, and a bottling plant. The stream may be split and sent to multiple recipients.

For this process, the cryogenic fluid is supplied in its gaseous form at near ambient temperature and is pre-cooled in heat exchange with the first stream of LNG; compressed using a compressor to supercritical pressure; sub-cooled in heat exchange with the refrigerant in the closed-loop refrigeration cycle; and expanded, whereby it condenses to form a cryogenic liquid.

The closed-loop refrigeration cycle is used to cool the cryogenic fluid to a temperature below the saturation temperature of LNG. The closed-loop system may be a single-phase Brayton cycle wherein the heat transfer fluid is compressed with a compressor; cooled in counter-flow heat exchange with the second stream of LNG; expanded in an expander; and warmed in heat exchange with the pre-cooled, compressed gaseous phase cryogenic fluid.

During export of LNG, the present invention uses some of the cold produced by re- gasification of the LNG to re-liquefy boil-off gas. The boil-off gas is compressed with a compressor; and cooled in counter-flow heat exchange with the refrigerant in the closed-loop refrigeration cycle, whereby its condenses into liquid phase.

During times of zero export of LNG (i.e. when no LNG is exported on the network), the present invention uses the cold stored in the cryogenic fluid to re-liquefy boil-off gas. Thus, the boil-off gas is compressed using a compressor; and cooled in heat exchange with the cryogenic fluid such that it becomes liquid. The warmed cryogenic fluid is thus vaporised, super-heated; and expanded isentropically through one or multiple turbo-expansion stages, thus producing work.

During times of low export of LNG, the present invention may use both the cold from the re- gasification of LNG and the cold stored in the cryogenic fluid to re-liquefy boil-off gas.

The system is able to operate flexibly, at different operating points, by altering the flow of boil-off gas (e.g. by changing the flow rate and/or by redirecting the boil-off gas as described below) and by adjusting the duty of the nitrogen and boil-off gas compressors accordingly.

A cryogenic store (e.g. storage tank) is provided for storing the cryogenic fluid, allowing the flow of cryogenic fluid in and the flow of cryogenic fluid out to be controlled independently. Thus, the heat transfer rate between the cryogenic fluid and the LNG, and the heat transfer rate between the boil-off gas and the cryogenic fluid from the cryogenic storage tank, may be independently and dynamically controlled by varying the flow rate of the cryogenic fluid into and the flow rate of the cryogenic fluid out of the cryogenic storage tank respectively. The re-gasification of LNG and the re-liquefaction of boil-off gas may therefore occur independently at different times and at different rates.

As a skilled person will recognise, the larger the volume of the cryogenic storage tank, the longer the period that boil-off gas may be re-liquefied during times of low, or zero, LNG send-out.

The flow rates may be controlled in response to both current, real time operational parameters and future predicted operational parameters in order to optimise the management of the LNG stock in the LNG tank. Operational parameters include, for example, one or more of demand for LNG, availability of LNG or cryogenic fluid, and rate of boil-off

In one example, the flow rate of liquid cryogenic fluid out of the cryogenic storage tank may be controlled as a function of the measured flow of boil-off gas. Alternatively, if the period of low, or zero, LNG sendout is predicted to be short, it may be preferential to economise the stock of liquid cryogenic fluid in the cryogenic storage tank and allow boil-off gas to accumulate within the pressure limits of the LNG tank.

In another example, the flow-rate of gaseous cryogenic fluid may be controlled as a function of the LNG sendout rate. Alternatively, it may be reduced as the cryogenic storage tank approaches full capacity. In one embodiment, during LNG send-out, boil-off gas may be mixed in its gaseous phase with the gasified liquid natural gas rather than being re-liquefied.

Turning to the system diagram shown in Fig. , the cold boil-off gas, which comes from an LNG tank or a chamber, vessel, header or anywhere where boil-off gas is collected, is withdrawn via conduit 1 by compressor 3. Boil-off gas is compressed into conduit 2 from the tank storage pressure, which normally is just above ambient pressure, to between 1 and 10 bar, but more typically 3 to 6 bar. At times of high LNG send-out rate no fraction of boil-off gas is diverted into conduit 42 but it is all conveyed through conduit 4 and liquefied and sub-cooled in heat exchanger 5. Boil-off gas, which is now in its liquid form, thus can be used as LNG, is then expanded through an expansion device 7, and conveyed by pump 9 to an LNG tank 11 via conduit 10.

Nitrogen, in gaseous form, available at a pressure between 1 and 16 bar, but more typically 6 to 9 bar, is withdrawn via conduit 12 and passed through heat exchanger 13 where it is cooled to near LNG storage temperature. Nitrogen is then compressed by a single or multistage compressor 15 to a pressure between 50 and 70 bar, but more typically 54 to 60 bar. Nitrogen, which is now above its supercritical pressure, is cooled in heat exchanger 5 to between -155°C and -185°C, but more typically -165°C and -175°C. Leaving the heat exchanger the nitrogen passes through conduit 21 and then expands through the expansion device 22. The liquid fraction obtained from the isenthalpic expansion, which in this embodiment is 100%, passes through conduit 23 to reach the liquid nitrogen storage tank 24.

Cooling to heat exchanger 5 is supplied by the refrigeration cycle shown between heat exchangers 5 and 29, where a refrigerant gas, typically nitrogen, is compressed by compressor 37 to between 4 bar and 16 bar, but more typically 7 bar to 10 bar, fed to heat exchanger 29, wherein it is cooled by heat exchange with LNG to between -161 °C and -140°C, but more typically -156°C. The cold refrigerant passes through conduit 39 to reach the inlet of the expansion device 40, where the refrigerant is expanded to between 1 bar and 7 bar, but more typically 2 to 4 bar. The refrigerant passes through conduit 41 and is fed to heat exchanger 5 at a temperature between - 190°C and -170°C, more typically -185°C.

Cooling to heat exchangers 29 and 13 is supplied by the LNG which is withdrawn from the LNG tank 1 1 by the LNG pump 26, pumped to a pressure between 60bar and 150 bar, more typically 80 bar and 120 bar. The high pressure LNG in conduit 27 is then split in two streams. A proportion of the LNG flow is directed to heat exchanger 29 via conduit 28 and the rest is sent to heat exchanger 13 via conduit 32. Conduit 30 and 33 are merged together to form conduit 34 and convey the LNG, which is now in gaseous form, to the natural gas distribution network. Similarly to any other commodity, LNG is subject to a volatile demand which means that the send-out rate can vary between 0% and 100% of the maximum capacity of the LNG re-gasification terminal. When the send-out rate is above a certain threshold there is enough cold to liquefy the boil-off gas stream and the nitrogen stream. However when the send-out rate drops below this threshold it is enough to turn down the nitrogen compressor 15 to adjust the system to the new operating conditions. However the preferred system can easily adjust to any level of send-out rate by completely stopping compressor 15 and, should the LNG send-out rate drop even further, diverting some of the compressed boil-off gas to conduit 42. The boil-off gas is then conveyed to heat exchanger 43, wherein it is cooled, liquefied and subcooled by heat exchange with liquid nitrogen. Boil-off gas, which is now in its liquid form, is then expanded through an expansion device 45, and conveyed by pump 47 to an LNG tank 1 1 via conduit 48.

The liquid nitrogen flow rate which passes through heat exchanger 43 is throttled by control valve 50. The nitrogen emerges from heat exchanger 43 in conduit 52 in its gaseous form. The nitrogen is then superheated in heat exchanger 53 to any temperature up to 500°C and expanded through a turbine 55 to recover the energy. Depending by the pressure and type of machine employed the expansion of the nitrogen stream can be done in a single stage, two stages, as shown in Fig. , or several stages with intermediate heat exchangers for superheating the nitrogen.

Control of the system is achieved using any conventional controller which operates to vary the duty of gaseous cryogenic fluid compressor 15 to control the flow rate of the stream of gaseous cryogenic fluid; open and close valve 50 to control the flow rate of the stream of liquefied cryogenic fluid from tank 24; and optionally vary the duty of gaseous boil-off gas compressor 3 to control the flow rate of the stream of gaseous boil-off gas. However, other means for controlling the flow rates of these streams are possible and within the capabilities of a skilled person to implement depending on particular circumstances.

A system diagram of a second embodiment of the invention is shown in Fig.2. The second embodiment is identical to the first in every way, except that the cryogenic fluid is air rather than nitrogen. Thus, conduit 12 no longer coveys gaseous nitrogen but ambient air which has undergone a cleaning, scrubbing and drying process. Ambient air is withdrawn through conduit 61 , it undergoes a first stage of cleaning as it passes through the air filter 62, compressed by compressor 64, sent to the air filtration unit 66, where moisture, carbon dioxide and hydrocarbons are removed, before emerging as clean and dry air in conduit 12. A system diagram of a third embodiment of the invention is shown in Fig.3. The third embodiment is identical to the first in every way, except that the liquid fraction obtained from the isenthalpic expansion of the nitrogen is not 100%, resulting in a vapour or gas phase of nitrogen existing immediately upstream of the nitrogen tank 24. Thus, in this embodiment, a separator 17 is added between the tank 24 and the expansion device 22. The liquid and the vapour fraction obtained from the isenthalpic expansion passes through conduit 23 to reach the separator 17, wherein the liquid fraction is conveyed via conduit 18 to the nitrogen storage tank 24 and the vapour fraction is conveyed via conduit 19 to heat exchanger 5. The nitrogen is warmed by heat exchange with incoming warm nitrogen and boil-off gas in heat exchanger 5 and then conveyed via conduit 20 back to the suction side of compressor 15 where it joins the incoming nitrogen in conduit 12.

A system diagram of a fourth embodiment of the invention is shown in Fig.4. The fourth embodiment is identical to the first in every way, except that a pump 35 is installed downstream of the control valve to raise the pressure of the liquefied nitrogen from the nitrogen tank to between 100 bar and 200 bar, but more typically 120 bar and 150 bar. The nitrogen emerges from heat exchanger 43 at high pressure and enters conduit 52 in its gaseous form. The nitrogen is then superheated in heat exchanger 53 to any temperature up to 500°C and expanded through a turbine 55 to recover the energy. Depending on the pressure and type of machine employed the expansion of the nitrogen stream can be done in a single stage, two stages, as shown in Fig.4, or several stages with intermediate heat exchangers for superheating the nitrogen. In this embodiment the expansion turbines would be able to generate more power per unit mass of nitrogen compared to the first embodiment of this invention but a higher flow rate of nitrogen would be required to liquefy the same boil-off gas flow rate.

A system diagram of a fifth embodiment of the invention is shown in Fig.5. The fifth embodiment is identical to the first in every way, except that heat exchanger 5 and heat exchanger 43 from previous embodiments are replaced with a single heat exchanger 70. In this embodiment the system no longer needs a separate heat exchanger to liquefy the boil-off gas when using liquid nitrogen.

It will be understood that modifications can be made to the methods and systems described herein without departing from the present invention which is defined by the appended claims.

Claims

1. A method for liquefying boil-off gas, comprising:

storing a liquefied hydrocarbon gas in a liquefied hydrocarbon gas store;

a) the stream of liquefied hydrocarbon gas undergoes a phase change from a liquefied hydrocarbon gas to a gaseous hydrocarbon gas; and b) the stream of gaseous cryogenic fluid undergoes a phase change from a gaseous cryogenic fluid to a liquefied cryogenic fluid;

wherein the step of processing comprises transferring heat from the stream of gaseous cryogenic fluid to the stream of liquefied hydrocarbon gas from the liquefied hydrocarbon gas store;

storing the liquefied cryogenic fluid in a liquefied cryogenic fluid store;

a) the stream of liquefied cryogenic fluid undergoes a phase change from a liquefied cryogenic fluid to a gaseous cryogenic fluid; and b) the stream of gaseous boil-off gas undergoes a phase change from a gaseous boil-off gas to a liquefied boil-off gas;

storing the liquefied boil-off gas in the liquefied hydrocarbon gas store;

independently controlling the flow rate of the stream of liquefied cryogenic fluid from the liquefied cryogenic fluid store based at least in part on the flow rate of the stream of gaseous boil-off gas.

2. The method of claim 1 , further comprising the step of processing the stream of gaseous boil-off gas and the stream of liquefied hydrocarbon gas from the liquefied hydrocarbon gas store such that:

3. The method of claim 2, wherein the steps of:

a) transferring heat from the stream of gaseous cryogenic fluid to the stream of liquefied hydrocarbon gas from the liquefied hydrocarbon gas store; and

b) transferring heat from the stream of gaseous boil-off gas to the stream of liquefied hydrocarbon gas from the liquefied hydrocarbon gas store;

are concurrent.

4. The method of claim 2, wherein the steps of:

are not concurrent.

5. The method of any preceding claim, wherein the step of processing the stream of gaseous cryogenic fluid and the stream of liquefied hydrocarbon gas further comprises the step of:

expanding the stream of gaseous cryogenic fluid after heat transfer.

6. The method of any preceding claim, wherein the step of processing the stream of gaseous cryogenic fluid and the stream of liquefied hydrocarbon gas further comprises the step of:

compressing the stream of gaseous cryogenic fluid prior to heat transfer.

7. The method of claim 6, wherein the step of compressing the stream of gaseous cryogenic fluid prior to heat transfer comprises compressing the stream to a supercritical pressure.

8. The method of claim 6 or claim 7, further comprising the steps of passing the stream of liquefied hydrocarbon gas through first and second branches; wherein the step of transferring heat from the stream of gaseous cryogenic fluid to the stream of liquefied hydrocarbon gas from the liquefied hydrocarbon gas store further comprises:

transferring heat to a stream of liquefied hydrocarbon gas in the first branch from the stream of gaseous cryogenic fluid prior to compression; and transferring heat to a stream of liquefied hydrocarbon gas in the second branch from the stream of gaseous cryogenic fluid after compression.

9. The method of claim 8, further comprising combining the streams of gaseous hydrocarbon gas in the first and second branches.

10. The method of any preceding claim, further comprising the step of delivering the stream of gaseous hydrocarbon gas to a recipient.

1 1. The method of claim 10, wherein the recipient is one or more of: a hydrocarbon pipe network; a power station; and a consumer of gaseous hydrocarbon gas.

12. The method of any preceding claim, further comprising the step of collecting the stream of gaseous boil-off gas.

13. The method of claim 12, wherein the step of collecting the stream of gaseous boil-off gas comprises collecting the boil-off gas from the liquefied hydrocarbon gas store.

14. The method of claim 12 or claim 13, wherein the step of collecting the stream of gaseous boil-off gas comprises collecting the boil-off gas from a store, conduit, or collection point coupled to the liquefied hydrocarbon gas store.

15. The method of any preceding claim, wherein the step of transferring heat from the stream of gaseous cryogenic fluid to the stream of liquefied hydrocarbon gas from the hydrocarbon gas store further comprises:

transferring heat from the stream of gaseous cryogenic fluid to a heat transfer fluid in a closed-loop refrigeration circuit and cooling the gaseous cryogenic fluid to a temperature below the saturation temperature of the liquefied hydrocarbon gas; and transferring heat from the heat transfer fluid in the closed-loop refrigeration circuit to the stream of liquefied hydrocarbon gas.

16. The method of claim 15, when dependent on claim 2 or any claim dependent thereon, wherein the step of transferring heat from the stream of gaseous boil-off gas to the stream of liquefied hydrocarbon gas from the liquefied hydrocarbon gas store further comprises:

17. The method of any preceding claim, further comprising processing a stream of ambient air to form the stream of gaseous cryogenic fluid.

18. The method of claim 17, wherein the step of processing the stream of ambient air comprises one or both of the steps of:

filtering the stream of ambient air to remove moisture, carbon dioxide and/or hydrocarbons; and

compressing the stream of ambient air.

19. The method of any preceding claim, further comprising passing the stream of liquefied cryogenic fluid through a separator prior to it entering the liquefied cryogenic fluid tank to separate any residual vapour phase from the stream of liquefied cryogenic fluid, and returning the residual vapour phase to the stream of gaseous cryogenic fluid.

20. The method of any preceding claim, further comprising pumping the stream of liquefied cryogenic fluid from the liquefied cryogenic fluid store to increase its pressure prior to the step of transferring heat from the stream of gaseous boil-off gas to the stream of liquefied cryogenic fluid from the liquefied cryogenic fluid store.

21. The method of any preceding claim, wherein the step of transferring heat from the stream of gaseous boil-off gas to the stream of liquefied cryogenic fluid from the liquefied cryogenic fluid store such that the stream of liquefied cryogenic fluid undergoes a phase change from a liquefied cryogenic fluid to a gaseous cryogenic fluid results in a second stream of gaseous cryogenic fluid, the method further comprising the step of expanding the second stream of gaseous cryogenic fluid to extract work from the stream.

22. The method of claim 21 , wherein the step of expanding the second stream of gaseous cryogenic fluid to extract work from the second stream is performed in a single- stage expansion device, a two-stage expansion device, or a multi-stage expansion device.

23. The method of claim 21 or claim 22, further comprising the step of super-heating the second stream of gaseous cryogenic fluid prior to one or more stages of expansion.

24. The method of any one of claims 21 to 23, further comprising the step of converting the work extracted from the second stream into electricity.

25. A system for liquefying boil-off gas, comprising:

a first store for storing liquefied hydrocarbon gas;

a second store for storing a liquefied cryogenic fluid;

a controller configured to:

a) control the flow rate of the stream of gaseous cryogenic fluid passing through the third arrangement of conduits based at least in part on the flow rate of the stream of liquefied hydrocarbon gas passing through the first arrangement of conduits; and

b) independently control the flow rate of the stream of liquefied cryogenic fluid passing through the fourth arrangement of conduits based at least in part on the flow rate of the stream of gaseous boil- off gas passing through the second arrangement of conduits.

26. The system of claim 27, wherein the first and second arrangements of conduits are arranged such that heat is transferred from the stream of gaseous boil-off gas passing through the second arrangement of conduits to the stream of liquefied hydrocarbon gas passing through the first arrangement of conduits.

27. The system of claim 25 or claim 26, wherein the third arrangement of conduits comprises a compressor for compressing the stream of gaseous cryogenic fluid; and wherein the first arrangement of conduits comprises a first branch and a second branch;

the first branch being arranged such that heat is transferred to a stream of liquefied hydrocarbon gas passing through the first branch from the stream of gaseous cryogenic fluid passing through the third arrangement of conduits at a first heat exchange region upstream of the compressor; and

the second branch being arranged such that heat is transferred to a stream of liquefied hydrocarbon gas passing through the second branch from a stream of gaseous cryogenic fluid passing through the third arrangement of conduits at a second heat exchange region downstream of the compressor.

28. The system of claim 27, wherein the first and second branches bifurcate from a single conduit upstream of the first and second heat exchange regions, and recombine to a single conduit downstream of the first and second heat exchange regions.

29. The system of any one of claims 25 to 28, wherein the source of boil-off gas is the first store.

30. The system of any one of claims 25 to 28, wherein the source of boil-off gas is a store, conduit, or collection point coupled to the first store.

31. The system of any one of claims 35 to 30, wherein the first and third arrangements of conduits are arranged such that heat is transferred between the first and third arrangements of conduits via a closed-loop refrigeration circuit comprising a heat transfer fluid passing through a fifth arrangement of conduits, wherein:

the fifth and third arrangements of conduits are arranged such that heat is transferred from the stream of gaseous cryogenic fluid passing through the third arrangement of conduits to the heat transfer fluid passing through the fifth arrangement of conduits; and

the fifth and first arrangements of conduits are arranged such that heat is transferred from the heat transfer fluid passing through the fifth arrangement of conduits to the stream of liquefied hydrocarbon gas passing through the first arrangement of conduits.

32. The system of claim 31 , when dependent on claim 25, wherein the first and second arrangements of conduits are arranged such that heat is transferred between the first and second arrangements of conduits via the closed-loop refrigeration circuit, wherein:

the fifth and second arrangements of conduits are arranged such that heat is transferred from the stream of gaseous boil-off gas passing through the second arrangement of conduits to the heat transfer fluid passing through the fifth arrangement of conduits.

33. The system of claim 31 , when dependent on claim 27, wherein the second branch is arranged such that heat is transferred from the heat transfer fluid passing through the fifth arrangement of conduits to the stream of liquefied hydrocarbon gas passing through the second branch.

34. The system of any one of claims 25 to 33, wherein the stream of gaseous cryogenic fluid is air, and wherein the third arrangement of conduits further comprises one or both of:

a filtration system for removing moisture, carbon dioxide and/or hydrocarbons from a stream of ambient air; and

a compressor for compressing a stream of ambient air.

35. The system of any one of claims 25 to 34, wherein the third arrangement of conduits further comprises a separator upstream of the second store for extracting any residual vapour phase from the stream of liquefied cryogenic fluid passing through the third arrangement of conduits prior to entering the second store, and a return conduit arranged to direct the residual vapour phase extracted from the stream of liquefied cryogenic fluid to the stream of gaseous cryogenic fluid passing through the third arrangement of conduits.

36. The system of any one of claims 25 to 35, wherein the second and fourth arrangements of conduits are arranged such that heat is transferred between the second and fourth arrangements of conduits at a third heat exchange region, and wherein the fourth arrangement of conduits further comprises a pump upstream of the third heat exchange region for pumping the stream of liquefied cryogenic fluid passing through the fourth arrangement of conduits prior to it passing through the third heat exchange region.

37. The system of claim 36, wherein the third heat exchange region is configured such that heat is transferred from the stream of gaseous boil-off gas passing through the second arrangement of conduits to the stream of liquefied cryogenic fluid passing through the fourth arrangement of conduits to produce a second stream of gaseous cryogenic fluid, and wherein the fourth arrangement of conduits further comprises an expansion device for expanding the second stream of gaseous cryogenic fluid and extracting work from the second stream of cryogenic fluid.

38. The system of claim 37, wherein the expansion device is a single-stage expansion device, a two-stage expansion device, or a multi-stage expansion device.

39. The system of claim 37 or 38, wherein the fourth arrangement of conduits is coupled to one or more super-heaters, wherein each super-heater is either upstream of the first stage of the expansion device or between stages of the expansion device.

40. The system of any one of claims 25 to 29, wherein the first, second, third and fourth arrangements of conduits are arranged such that heat is transferred between the first and third arrangements of conduits, between the second and fourth arrangements of conduits at a single heat exchange region.

41. The system of claim 40, when dependent on claim 26 or any claim dependent thereon, wherein the first, second, third and fourth arrangements of conduits are arranged such that heat is transferred between the first and second arrangements of conduits at the single heat exchange region.

42. The system of claim 31 or the method of claim 15, or any claim dependent thereon, wherein the closed-loop refrigeration circuit operates using one of a single- phase Brayton cycle and a dual-phase Rankine cycle.

43. The system of claim 31 or the method of claim 15, or any claim dependent thereon, wherein the heat transfer fluid is one of nitrogen or propane.

44. The system or method of any preceding claim wherein the cryogenic fluid is one of nitrogen or air, preferably ambient air.

45. The system or method of any preceding claim wherein the liquefied hydrocarbon gas is Liquefied Natural Gas (LNG).

46. A method substantially as described herein with reference to the accompanying drawings.

47. A system substantially as described herein with reference to and as shown in the accompanying drawings.