ABSTRACT
One of the major advantages of using steam propulsion plant in LNG carriers has been its capability to burn the unavoidable BOG directly in the power boiler, which has made it practically an exclusive option for many years. However, as environmental, economic and technical expectations have increased, the drawbacks of the steam turbine power plant have made it a less attractive option. Among these drawbacks are the comparative low efficiency of the plant, its high fuel consumption, which in turn translates into high exhaust emissions and its large engine room space requirement. Advances in the design of dualfuel diesel engines, shipboard LNG reliquefaction plants and marine gas turbines, provide meaningful alternatives to the traditional steam power plant for LNG vessels. (Gilmore et al. 2005)
Regarding to environmental concern, controls on exhaust gas emissions continue to tighten regionally and internationally dictating further responses from engine designers. Carbon dioxide (CO2), nitrogen oxides (NOX), sulphur oxides (SOX) and Particulate Materials (PM) are the gaseous emissions of most concern. Thereby, in-engine measures to decrease these emissions, including common rail fuel systems, emulsified fuel, direct water injection and charge air humidification, have been studied. In addition, exhaust gas after-treatment, such as Selective Catalytic Reduction (SCR) and Exhaust Gas Recirculation (EGR)
1 INTRODUCTION
LNG carriers are specialized ships designed to transport Liquefied Natural Gas (LNG).They are fitted with insulated double-hulled tanks, designed to contain the cargo slightly above atmospheric pressure at a cryogenic temperature of approximately −169°C. An average LNG carrier presents tank capacity about 160,000 m3 and typically, the storage tanks operate at 0.3 barg with a design pressure of 0.7 barg. LNG presents typically density between 430 and 470 kg/m3, depending on its composition and state. The LNG is composed predominantly by methane (CH4), as well as ethane (C2H6), propane (C3H8), butane (C4H10) and nitrogen (N2). (Mokhatab et al. 2014)
Despite the high degree of insulation in the tank walls, it is impossible to avoid the heat transfer from the surroundings, so that some vaporization will be always present during LNG transportation by ships. This occurs mainly due the thermal conductivity through the tank walls and the movement of the liquid. That LNG evaporated is called Boil-off Gas (BOG) and its evaporation rate is called Boil-off Rate (BOR). The natural BOR from a typical LNG carrier tank is about 0.10 to 0.15% in volume per day, depending on the thermal insulation system (Mokhatab et al. 2014). Vaporization induces an increase in pressure in the tank, such that a certain amount of the vapour phase should be taken out of the tank to avoid dangerous overpressure. Usually, this outlet gas flow is used as fuel
systems, as well as gas scrubbers, also have been developed for this purpose. (Woodyard 2009)
Particularly regarding to CO2 emissions reduction, the solutions are mainly burning alternative fuels, as BOG for instance, and decreasing the fuel consumption. The latter is achieved by means of the increasing of the ship energy efficiency. Measures that positively affect it include the improved hull and ship structure designs, which result in decreased ship resistance, as well as more efficient propulsors designs that can increase the ship propulsive efficiency (IMO 2011). Further, more efficient propulsion types and the exploitation of the rejected energy as much as possible have been pursued. In this sense, waste heat recovery systems, including Power Turbine Generation (PTG) and Steam Turbine Generation (STG), have emerged.
