【作者】

Martijn van Essen

【摘要】

该论文已在赫尔辛基举行的第28届CIMAC大会上发表，论文的版权归CIMAC所有。Liquefied natural gas (LNG) is becoming an attractive alternative to traditional transportation fuels such as diesel and heavy fuel oil. The major advantages in terms of reduced pollutant emissions and its worldwide availability make it an attractive fuel for powering ships. This has resulted in a rapidly growing number of LNG-fuelled ships. The natural boiloff gas (NBOG), which is taken from the top of the tank, can directly be used as fuel gas for the engines. The NBOG typically has a high methane content and hence a high knock resistance. However, when the power demand is higher than the power that can be generated using NBOG, forced boil-off gas (FBOG, obtained by vaporizing liquid LNG) is mixed with the NBOG. Gas from forced boil off contains all the hydrocarbons present in the LNG and usually has a lower knock resistance than that produced by natural boil off. Thus, adding FBOG to NBOG reduces the knock resistance of the fuel and appropriate precautions should be taken in situation when the knock resistance of the LNG is low to avoid the occurrence of engine knock. Mild engine knock increases pollutant emissions and can damage the engine in the long term, while severe knock can physically destroy the engine in seconds. Thus, engine knock should be avoided. There is a wide variety of engine types used in LNG-fuelled ships. Both engine manufacturers and fleet owners must be certain that the engines chosen can accept the range of LNG compositions available in the ports at which they bunker. To ensure that the engines to be used in LNG-fuelled ships are matched with the expected variations in fuel composition, the knock resistance of the LNG fuels must be determined, and subsequently specified, unambiguously. Recently, DNV GL developed a method to characterize gases with respect to their knock resistance. The DNV GL method is based on the physics and chemistry of combustion, and is uniquely flexible in being adaptable for any engine type and gas composition. Furthermore, it has been demonstrated to have superior accuracy for predicting the knock resistance for different gas compositions than the existing methods. In addition, to be able to predict the effect of boil off on the knock resistance of NBOG and FBOG of typical LNGs during a voyage, we coupled the knock model to a dynamic model that computes the evaporation of LNG in tanks. This model is part of the DNV GL COSSMOS modeling framework for ship machinery systems. The model employs coupled thermodynamic non-linear vapourliquid phase equilibrium equations and differential conservation equations describing the evolution of LNG quantity and composition with time. The boil-off model is used to calculate the variation in composition of the LNG and the boiloff gas, during typical voyage profiles derived from a case study LNG vessel. The gas compositions calculated from the boil-off model are then used as input to the knock model to calculate the knock resistance of the LNG (thus, the FBOG) and NBOG during the voyage. The results provide valuable input and insight regarding the acceptability of the range of LNG qualities in the ports at which ships bunker.

【会议名称】

第28届CIMAC会议

【会议地点】

芬兰 赫尔辛基

【下载次数】

2

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