Abstract
Human efforts to devise optimum propulsion for their vessels are as old as the vessels themselves. Today these efforts are even more determined as modern shipping requires propulsion systems that are increasingly reliable, available, cost-efficient and able to meet high ecological criteria. The heat transfer towards liquefied gas stored in tanks results in boil-off during cargo handling or voyage. The rate of the evaporated gas amounts to 0.13% per day during the voyage of a fully loaded ship. Steam turbines have been a dominant form of propulsion on liquefied natural gas - LNG carriers for over forty years. Until recently, the possibility of using boil-off gas as fuel for boilers has been the reason for installing steam plants as the only means of propulsion of LNG carriers. However, it has been proved that these plants are not sufficiently efficient due to adverse impacts on both emissions and the vessel’s operating expenses. It has also been found out that dual-fuel-electric propulsion is the most effective alternative to steam. Shipping companies select electric propulsion primarily because it provides excellent manoeuvrability and increased availability, allows reduction of the machinery space and better arrangement of shipping capacity and, naturally, because of lower fuel costs. This paper discusses the newest technologies and the operation principle of the low-pressure four-stroke dual-fuel diesel engine, specificallythe 12V50DF and 9L50DF types produced by Wärtsilä company, and the concept of the dual-fuel-electric propulsion for the new generation of LNG carriers.
Highlights
Dual fuel diesel engines are the engines that can burn natural gas in addition to marine diesel oil or heavy fuel oil
The evaporated gas from the cargo tanks is conveyed under pressure through the boil-off heater and natural boil-off mist separator (NBO) to the engine by means of centrifugal twostage LD compressors installed in the maindeck cargo handling room, at nominal temperature of 30 °C, according to (Machinery operating manual, 2009)
Prior to entering the engine, Boil-off gas (BOG) passes through the gas valve unit (GVU) which consists of a filter, temperature sensor, pressure sensor, pressure regulating valve, two safety tripping valves, and three remotely-controlled ventilation regulating valves, see Figure 8, according to (Machinery operating manual, 2009; Norrgärd, 2006)
Summary
Dual fuel diesel engines are the engines that can burn natural gas in addition to marine diesel oil or heavy fuel oil. The amount of nitrogen in BOG may amount up to 30 % in volume at the beginning of the loaded ship voyage The engines and their fuel control systems are designed to adjust to all conditions and to enable each engine to be running on gas at their normal rating without evaporisation, according to (Machinery operating manual, 2009). The engines are fully capable of switching over from one fuel to another during operation and when running under load, without interrupting the electric power supply These engines are identical with regard to their components such as pistons, valves, injectors, etc. Gas ignites and burns, pushing the piston downwards as in a conventional diesel engine where power is developed by liquid fuel combustion
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