Dynamic simulation and process control study for enhancing operability of small-scale BOG (Boil-off gas) re-liquefaction process in LNG-fueled ship

Abstract

Abstract There has been an increasing demand for natural gas because it emits less greenhouse gases than conventional fossil fuels. Reinforcement of international regulation to reduce exhaust gas from ship also supports to fuel a ship with LNG (Liquefied natural gas). However, as LNG stored in a tank at low temperature keeps losing its heat into surroundings, generation of BOG (Boil-off gas) occurs. This necessitates the introduction of BOG re-liquefaction process because the accumulation of BOG not only leads to abnormal state in LNG tank but also decreases profitability. While previous studies on BOG re-liquefaction processes are mostly limited to large-shipping capacity for LNG carriers using SS (steady state) simulation, this research aims at developing a novel framework for operational optimization of small-scale BOG re-liquefaction process equipped onto LNG-fueled ship by considering both SS and DS (dynamic state) simulation. For this purpose, the process is first designed with small-scale N2-based reverse Brayton cycle which can be operated at offshore environments. Its feasibility is verified with SS simulation in a commercial simulator. Dynamic model developed in this study shows good agreement with SS simulation, indicating that the model is suitable for analyzing dynamic behaviors and control systems. Sensitivity analysis based on the SS simulation is also carried out for key design and operating parameters with which operation strategy for dealing with unexpected changes is obtained and guidance for achieving more economic operation, leading to the reduction of shaft power consumption for the cycle, is gained. Process dynamics can be better predicted with DS model built in this study and robust control is effectively ensured with the proposed control logic. This improvement is essential to prevent unexpected operational failure (e.g. surging and thermal shock) or unwanted deterioration in system performance, due to disturbance in the feed and operating conditions. There are various control mechanisms available for BOG re-liquefaction process. For example, LNG temperature can be controlled by either feed-backward or feed-forward way and faster dynamic response can be achieved with feed-forward way. The most robust control scheme which has stability and high energy efficiency is chosen such that manipulated and controlled variables are paired in a logical manner followed by investigating impacts of disturbances, like change in feed gas temperature. Furthermore, various operational scenarios are systematically investigated in the context of HAZOP (Hazards and operability study), where DS simulation is utilized for optimizing system performance or minimizing energy consumption. In conclusion, practical and cost-effective operational guidelines are systematically obtained through the proposed operating strategy and control system.