NATURAL GAS UTILIZATION FOR SHIP PROPULSION

About 3% of the global emissions are produced by ship transport but are concentrated in coastal and small areas with high ship traffic. For these reasons, the International Maritime Organization (IMO) has developed a program for reducing the environmental impact of the maritime transport. In order to respect these new rules, natural gas used as ship fuel is an interesting solution since it allows to reduce pollutant emissions and it is available in large quantities and at a price today lower than diesel oil. To identify the most suitable vessels for an LNG propulsion project, a statistical analysis of the world maritime traffic has been carried out. The most suitable vessels has been selected on the basis of their time spent into the Emission Controlled Area (ECA, area with strict limits on gas emissions) and the delivery forecast of new ship. Furthermore, a second analysis using traffic data of three different years has been developed to confirm the robustness of the obtained results and highlight possible trends in the considered period. The results pointed out that tanker ships are one of the best ship type for the LNG propulsion. The use of natural gas in its liquid form need gas engines, cryogenic tank and a gas handling system: all these components have been described highlighting advantages and drawbacks. To assess the technical feasibility and highlight the main issues, dynamic simulations on the gas tank of specific ships have been developed. In particular, tank pressure variation and gas composition has been examined since influences the gas plant layout and the engine performances. The analysis highlighted that the tank insulation is one of the most critical parameters: further investigations are necessary to develop a control strategy aimed to control the tank pressure maintaining a good quality of the gas mixture. Furthermore, the use of natural gas allows new energy recovery technologies: an energy analysis on a specific ship has been carried out to demonstrate the possible efficiency improvements and then fuel and emissions savings. The analysis encompasses several recovery technologies, starting from the simple heat recovery to more complex systems with Organic Rankine cycle adoption coupled with heat recovery. An efficiency increasing of about 5% respect to a traditional tanker ship with heat recovery has been obtained. The last objective is to demonstrate the environmental and economic benefit of the LNG propulsion: for this reason a comparison between oil fuel and natural gas has been carried out, highlighting the gas emissions savings and the payback periods of the different plant solutions in compliance with the new emissions rules. Different plant solutions and price scenarios has been considered to verify also the reliability of the obtained results. The results demonstrated that a CO2 emission reduction greater than 20% can be achieved and the payback period for an LNG system can be three years.