Responding to low-carbon shipping via a novel BOG-ORC-CCS system onboard LNG carrier: 4E analyses and optimization

As the international LNG trade market is booming, the LNG carrier fleet has expanded year after year. How to reduce energy consumption in boil-off gas (BOG) re-liquefaction process and CO₂ generated during transportation has become a hot topic. This paper obtains ideas from the LNG cold energy contained in LNG carriers, and proposes a novel BOG-ORC-CCS system for direct BOG re-liquefaction, Organic Rankine cycle (ORC) and carbon capture and storage (CCS). The BOG-ORC-CCS system recycles waste heat from the ship while realizing the LNG cold energy gradient utilization. The reference ship (Q-Flex) is specified under the framework of Energy Efficiency Design Index (EEDI) phase Ⅲ requirement and the BOG-ORC-CCS system is simulated by Aspen HYSYS software. A comprehensive evaluation of the system performance is carried out under the guidance of energy, exergy, economic and environment (4E) modeling. The results of the 4E analyses show that the proposed BOG-ORC-CCS system has the advantage of low energy consumption and good economic performance. Finally, a multi-objective optimization of the system performance in 4E aspects: system energy and exergy efficiency (η_en,sys×η_ex,sys), payback period (PBP) and primary energy saved (PES) are carried out by the third-generation non-dominated sorting genetic algorithm (NSGA-Ⅲ). As a result, the optimal solutions are as follows: η_en,sys is 50.91%, η_ex,sys is 54.00%, PBP is 1.55 years and PES is 1.32 MWh, while Load of 75%, T_L5 is −136.01 °C, p_B5 is 1.82 MPa and Scr_L2 is 99.88%. Additionally, SEC is 0.20 kWh/kg, which is much lower than that of conventional indirect re-liquefaction. This demonstrates the feasibility of the BOG-ORC-CCS system on LNG carriers.