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Marine LNG Engine
A marine LNG engine is a dual fuel engine that uses natural gas and bunker fuel to convert chemical energy in to mechanical energy. Due to natural gas’ cleaner burning properties, the use of natural gas in merchant ship propulsion plants is becoming an option for companies in order to comply with IMO and MARPOL environmental regulations. The natural gas is stored in liquid state (LNG) and the boil-off gas is routed to and burned in dual fuel engines.〔Fabio Burel, Rodolfo Taccani, Nicola Zuliani, Improving sustainability of maritime transport through utilization of Liquefied Natural Gas (LNG) for propulsion, Energy, 57, (1) August 2013, 412-420, http://dx.doi.org/10.1016/j.energy.2013.05.002.〕 Shipping companies have been cautious when choosing a propulsion system for their fleets. The steam turbine system has been the main choice as the prime mover on LNG carriers over the last several decades. The decades old system on steam propelled LNG carriers uses BOG (boil-off gas). LNG carriers are heavily insulated to keep the LNG at around -160°C – to keep it liquefied. What happens is that even with all the insulation, the LNG containment area is penetrated by heat which allows for naturally generated boil-off gas (BOG).〔Daejun Chang, Taejin Rhee, Kiil Nam, Kwangpil Chang, Donghun Lee, Samheon Jeong, A study on availability and safety of new propulsion systems for LNG carriers, Reliability Engineering & System Safety, 93, (12), December 2008, 1877-1885, http://dx.doi.org/10.1016/j.ress.2008.03.013.〕
==Boil-off gas==
The natural gas that fuels dual fuel engines is carried on ships as a boiling liquid, and transported at slightly higher than atmospheric pressure. When tank insulation is penetrated by any influx in heat, it will cause the temperature of the liquefied natural gas to rise, which allows for vaporization from liquid to gas. When heat penetrates the tank, the tank’s pressure increases due to boil-off. The insulation of the tanks is designed with the most advanced technology. Even still, the insulation of the tanks is penetrated by heat. The boil-off occurs during the ships voyage. During a storm, the LNG cargo moves and sloshes around in the tanks. The boil-off gas represents 0.1% - 0.25% of the ships capacity per day. Tanks need to be maintained at a steady pressure. If the pressure in tanks is not controlled relief or safety valves are forced to open, venting the boil-off into the atmosphere until pressure is relieved. At this point, it has been proven that onboard LNG reliquefaction is uneconomical for most ships. Instead, the gas produced by this boil-off effect is routed to the ship’s propulsion system and used as fuel for power plants such as steam boilers and duel fuel marine diesel engines. This reduces the use of bunker fuel, reducing fuel costs and equipment maintenance costs.〔Tusiani, M. D., & Shearer, G. (2007). LNG: A nontechnical guide. Tulsa, Okla: PennWell.〕
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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