Liquified natural gas (LNG) fuel has received significant attention as an affordable and highly efficient fuel option due to strengthened regulations on the sulfur content of bunker oil put in place by the International Maritime Organization. The main component of the LNG fuel gas supply system (FGSS) is the heat exchanger that provides adequate gas temperatures and pressures required by the engine, which also has a large machinery volume compared with other equipment. Due to the volume limitation, most FGSS have been applied to new shipbuilding only. To reduce the volume of the FGSS, CO2 was considered to serve as the replacement heat medium for conventionally used glycol water during LNG gasification. The specific power consumption (SPC) in the CO2 and glycol water system was optimized using the Aspen HYSYS thermodynamic modeler toward adjusting the temperature and pressure, and the resulting sizes were compared. This study demonstrated that the CO2 heat medium resulted in a 14% improvement in efficiency and a 7% reduction in heat exchanger size concluding that it was the most advantageous heat medium for the LNG regasification.
This study proposed the integrated design of an NH3 fuel supply system and a re-liquefaction system for an ocean-going NH3-fueled ship. The target ship was a 14,000 TEU large container ship traveling from Asia to Europe. The NH3 fuel supply system was developed to feed the liquid fuel at 40 °C and 80 bar and cope with the re-circulated fuel with the sealing oil. Its power consumptions and SECs ranged from 56.4 to 157.5 kW and from 0.0063 to 0.009 kWh/kg, respectively. An onboard re-liquefaction system with a vapor compression refrigeration cycle was also designed to liquefy the BOG from the fuel tank. The re-liquefaction system’s exergy efficiency and SEC were 34.71% and 0.224 kWh/kg, respectively. The equipment with the most exergy destruction was the heat exchangers, accounting for 60% of the total exergy destruction. NPV analysis found that it is recommended to introduce the re-liquefaction system to the target ship. At the NH3 price of USD 250/ton, the reasonable cost of the re-liquefaction system is less than USD 1 million. According to LCC, NH3 fuel is economically feasible if the carbon tax is more than USD 80/ton and the NH3 price is around USD 250/ton.
Air-independent propulsion systems have improved the performance and decreased the vulnerability of underwater weapon systems. Reforming systems, however, generates large amounts of water and CO2. The recovery or separation of CO2, a residual gas component generated in vessels, entails considerable cost and energy consumption. It is necessary to understand the characteristics of the interaction between CO2 and seawater under the conditions experienced by underwater weapon systems to design and optimize a CO2 treatment process for dissolving CO2 in seawater. In this study, numerical analysis was conducted using the derived experimental concentration and MATLAB. The diffusion coefficient was derived as a function of temperature according to the CO2 dissolution time. Experiments on CO2 dissolution in seawater were conducted. The concentration of CO2 according to the reaction pressure and experimental temperature was obtained. The diffusion coefficient between CO2 and seawater was found to be 6.3 × 10−5 cm2/s at 25 °C and 7.24 × 10−5 cm2/s at 32 °C. CO2 concentration could be estimated accurately under vessel operating conditions using the derived CO2 diffusion coefficients. Optimal design of the residual gas treatment process will be possible using the derived seawater–CO2 diffusion coefficients under the actual operating conditions experienced by underwater weapon systems.
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