Impact of natural gas quality on engine performances during a voyage using a thermodynamic fuel system model.

Liquefied Natural Gas (LNG) as a fuel is seen as a solution to curb harmful emissions in shipping and its quick uptake is now significant. As heat leaks into the LNG fuel tanks, composition of the vapor and liquid phase change. This process, known as ageing, alters the fuel properties and can impact negatively the main engine performance. In this paper, a dynamical model to assess the methane number at engine inlet from the bunkered composition is presented. The fuel system is made of a 40 ft container tank and two heat exchangers. The model considers the two-phase real mixture in the tank and solves explicitly the heat and mass conservation laws. Then a strategy is developed to avoid "off-spec" methane number at engine inlet. The approach is tested on two bunkered compositions, one from Libya and one from Australia. The simulations have been conducted for a concept LNG/wind electric ship using multiple removable 40 ft container LNG tanks. Results show that with the proposed mixing strategy, the methane number remains above its limit value at engine inlet, insuring optimal operating conditions of engines. • A model to assess quality of natural gas at engine inlet over a voyage is proposed. • A control strategy maintains the methane number above 80 (optimal operating point). • The control strategy relies on Forced Boil-off Gas and composition mixing. • The case study is a concept ship using removable containerized LNG tanks. [ABSTRACT FROM AUTHOR]