Optimization of the propulsion plant of a Liquefied Natural Gas transport ship

Stricter emission regulations and variability of fuel prices pose the focus on the optimization of steam turbine based propulsion plants of Liquefied Natural Gas (LNG) ships. The efficiency of such a propulsion plant has been improved in this work by studying the introduction of reheating and preheating stages in the onboard regenerative Rankine cycle. A thermodynamic model of the propulsion plant has been developed from the facility diagrams, being validated afterwards with available experimental data from actual ship operation. The predictions of different scenarios obtained by the model when introducing modifications in the power propulsion cycle showed promising results. It was found that a combination of preheating and reheating stages was found to increase the cycle efficiency up to 33.71%, reducing fuel consumption in around 20 t/day and CO2 emissions in more than 20,000 t per year. An exergy analysis of the impact of cycle modifications and an economic assessment of the proposed investment plan were performed. It was found that the boiler was the main contributor to exergy destruction, fact that justifies the cycle modifications performed. The economic analysis of the investment plan of implementing the selected alternative provided benefits even in a conservative scenario, with an Internal Rate of Return higher than 12% and a Pay-Back Period less than 9 years for all the studied scenarios. In summary, a practical industrial application of thermodynamic and exergy analysis to the propulsion plant of a LNG ship has been shown, allowing an efficiency, economic and environmental improvement. •Development of a thermodynamic model of the ship propulsion plant.•Experimental validation with actual operation data from the ship.•Preheating and reheating effects on the cycle efficiency are assessed.•Cycle efficiency is improved, fuel consumption and CO2 emissions are reduced.•Exergy and economic assessment of the cycle improvement.