Method and apparatus for pre-heating lng boil-off gas to ambient temperature prior to compression in a reliquefaction system

The invention relates to the field of re-liquefaction of boil-off gases from liquid natural gas (LNG). More specifically, the invention relates to a method and an apparatus for pre-heating LNG boil-off gas (BOG) stream flowing from a reservoir in a reliquefaction system, prior to compression, and a method and an apparatus for cooling an LNG boil-off gas (BOG) stream in a reliquefaction plant.

A new generation of LNG vessels was established in association with the introduction of LNG reliquefaction systems (LNG RS). Prior to this, basically all LNG vessels were driven by steam turbines fuelled by boil off gases (BOG) evaporating from the cargo during transportation. In periods when the total amount of BOG was insufficient to cover the entire power demand, additional LNG had to be fed to the boilers through forced vaporizers.

The new LNG RS opened the possibility to collect, cool down and reliquefy all BOG and hence preserve the total cargo volume throughout the laden and ballast voyages. Conventional slow speed diesel engines, with high efficiencies compared to the steam turbines, could then be used for propulsion.

Several patents have described various aspects with such reliquefaction plants, and accordingly improvements to these. The prior art (e.g. Norwegian Patent Application No. 20051315 basically focuses on improvements of the nitrogen Brayton cycle and the utilization of cold nitrogen for pre-cooling. There is, however, a further need to improve the system in order to reduce the power demands.

Most of today\'s LNG vessels utilize low-temperature centrifugal BOG compressors to feed their boilers. Much of the reason for choosing low-temperature compression is that this will reduce the compressor size significantly compared to compression at ambient temperatures. The fan laws are applicable for centrifugal compressors, and show that a low suction temperature will ensure a higher pressure ratio per stage. The density of the gas will accordingly increase, the volume flow is reduced to a minimum, and the size and efficiency of the BOG compressors become more favourable.

Since there is no need to preserve the low temperature duty in the BOG stream—in fact the BOG is normally additionally heated before introduction to the boilers—the heat of compression is deliberately absorbed by the compressed gas without any means of heat rejection downstream the BOG compression.

The common practice of low.temperature BOG compression has been further applied to new BOG compressor designs, dedicated for operation towards LNG reliquefaction systems. From an energy point-of-view this results in inefficient operation, since the cooling cycle must be sized to remove the heat of compression from BOG compressors, in addition to the heat of evaporation and the superheating adsorbed in the cargo containment system.

Also, other problems arise when low-temperature BOG compression is applied. Since no aftercoolers (intercoolers) are employed, recycling at low capacities depend on temperature control upstream the BOG compressor. The cooling duty necessary for this purpose can be difficult to predict since it will depend much on the BOG compressor efficiency, which in turn depends on several properties of the processed stream. Using recondensed BOG to provide this cooling, also reduces the performance of the plant, measured in terms of power per unit reliquefied BOG returned to the tanks.

It is thus provided a method of A method of pre-heating LNG boil-off gas (BOG) stream flowing from a reservoir in a reliquefaction system, prior to compression, the method comprising heat exchanging the BOG stream in a first heat exchanger, against a second coolant stream having a higher temperature than the BOG stream, the method being characterized in that the second coolant stream is obtained by selectively splitting a first coolant stream into said second coolant stream and a third coolant stream, said third coolant stream being flowed into a first coolant passage in a reliquefaction system cold box, whereby the BOG has reached near-ambient temperatures prior to compression and heat exchange with low temperature BOG is done by optimising the split of the coolant in the first heat exchanger in order to minimize exergy losses, and thermal stresses in the cold box are reduced.

It is also provided a method for cooling an LNG boil-off gas (BOG) stream in a reliquefaction plant, the BOG flowing from a reservoir, the method comprising compressing the BOG; heat exchanging the compressed BOG against a coolant in a cold box; flowing substantially re-liquefied BOG from the cold box to the reservoir, characterized by prior to the compression step, pre-heating the BOG to substantially ambient temperatures, by heat exchanging the BOG with said coolant, said coolant prior to the heat exchange having a higher temperature than the BOG.

In one embodiment, the pressure of the reliquefied BOG between the cold box and the reservoir is controlled independently of the BOG compressor discharge pressure and the reservoir pressure, and the amount of vent gas generated and the vent gas composition thus may be controlled.

It is also provided an apparatus for cooling an LNG boil-off gas (BOG) in a reliquefaction system, comprising a closed-loop coolant circuit for heat exchange between a coolant and the BOG; a BOG compressor having an inlet side fluidly connected to an LNG reservoir; a cold box having a BOG flowpath with a BOG inlet fluidly connected to the BOG compressor outlet side; said BOG flowpath having outlet for substantially re-liquefied BOG, fluidly connected to the reservoir; said cold box further comprising coolant flowpaths for heat exchange between the BOG and the coolant; characterized by a first heat exchanger in the fluid connection between the reservoir and the BOG compressor inlet side, said first heat exchanger having a coolant path fluidly connected to the closed-loop coolant circuit, at a point downstream of the coolant circuit\'s compander aftercooler but upstream of the coolant flow paths in the cold box, whereby the BOG compressor receives BOG with temperatures near or at the system ambient temperatures.

In one embodiment, the invention provides a separator in fluid connection with the cold box outlet and with the reservoir, a first valve in the cold box outlet line and a second valve in a line connected to the reservoir, said separator also comprising a vent line (11), whereby the pressure in the separator may be controlled, and the amount of vent gas and the vent gas composition thus may be adjusted.