Analysis of cascade cycle and reverse Brayton cycle based bog reliquefaction systems for their suitability in LNG carrier ships with dual fuel engines.

Abstract

Liquefied Natural Gas (LNG) carrier ships with dual fuel engines use natural gas often by pumping and vaporizing LNG. The cold energy of LNG can be utilized to improve the performance of the reliquefaction system. Cascade cycle (using hydrocarbon or derivatives) and Reverse Brayton cycle (RBC) (using nitrogen) are compared for their suitability as reliquefaction systems using exergy analysis. To condense an equal quantity of boil-off gas (BOG), heat exchangers and exergy input required for RBC are higher. RBC performs better when cold is utilized by BOG stream rather than by N2 stream. For cascade system, LNG cold cannot be extracted gainfully if flow rate of LNG through the pump is very low. One may apply the results given in this paper to choose either RBC or cascade cycles for reliquefaction of BOG utilizing cold from LNG fuel to engine and meet the criteria of size, power consumption and safety.