Nowadays an increasing numbers of small-scale LNG projects are arising in the world beside very big plants in the Asia Pacific area. In Europe a new regulatory framework is preparing the ground for the design and construction of many distributed LNG plants, with an increasing interest on micro-scale plants with production below 40TPD. In a LNG storage tank, because of the inevitably heat incomes from the ambient part of boil-off gas is generated. Especially in small-scale plants and in storage facilities, boil-off phenomena are very important and several methods are currently adopted to minimize the boil-off and re-liquefy the vaporized natural gas.
Among the different technologies to carry out this process, Stirling engines used as cryocoolers prove to have a very interesting potential because they are able to achieve temperatures well below the liquefaction of the natural gas thus assuring the continuous re-liquefaction of the boil-off or the subcooling of the natural gas.
In this work, the authors first analyse the most critical aspects related to the thermodynamic and operation of the LNG plants and storage that give rise to the boiloff phenomena and then evaluate the potential for the adoption of Stirling cycle cryocoolers. In particular, two small-scale LNG storage tanks with capacity of 1’500 m3 and 10’000 m3 have been considered. Despite the BOG from loading and unloading of the tanks has not been taken into account, the analysis has proved that StirLNG-4 and StirLNG-16 cryocoolers respectively are able to manage the generated BOG. Hence, the analysis has shown that commercially available Stirling cycle cryocoolers could be
used to re-liquify the BOG generated in small-scale LNG plants thus enlarging the potential market of these machines
