Energy-efficient design and optimization of boil-off gas (BOG) re-liquefaction process for liquefied natural gas (LNG)-fuelled ship

Abstract

Liquefied natural gas (LNG) fuelled shipping systems adopts the boil-off gas (BOG) re-liquefaction process to maintain the pressure of storage tank and to minimize methane loss. However, most of previous studies on on-board BOG re-liquefaction process were made for large-scale applications of LNG carriers. This paper focuses on small-scale BOG re-liquefaction process used for LNG fuelled ship. In order to improve energy efficiency of BOG re-liquefaction, process design and optimization study are carried out for the refrigeration process in LNG fuelled ship. The reverse Brayton cycle using nitrogen as a refrigerant fluid is selected and two different configurations without using cryogenic compressor are considered, namely, i) re-liquefaction of BOG feed stream without compression and ii) pre-heating of BOG feed to use compressor operating in ambient temperature. Thermodynamic analysis provides conceptual insights into the key operating variables on the performance of BOG re-liquefaction process, while energy-efficient strategy for achieving minimum power consumptions can be systematically obtained through process optimization. Sensitivity analysis is also performed to understand how the variation of operating conditions affects on system performance of BOG re-liquefaction process under different design conditions and constraints. The case study also illustrates how process modeling and optimization framework proposed in this paper can be effectively utilized to improve energy efficiency in BOG re-liquefaction process.