In recent years, there has been a significant increase in the level of interest on environment friendly and economically viable solutions for the transport of Liquefied Natural Gas (LNG).
It has passed more than 40 years since the first commercial export of LNG in the world. Utilization of LNG has been rapidly spreading in recent years owing to the growing energy needs of worldwide in the world market.
The growth of traded LNG volume is the highest of all fuels thanks to the environment-friendly characteristics of natural gas and the transport ability of LNG.
LNG carriers have, up to 2006, mainly been driven by steam turbines. The Boil-Off Gas from the LNG cargo has so far been used as fuel. This is a costly solution that requires special skills during construction and operation. Alternative propulsion systems offer far better fuel economical efficiency than steam turbines. Instead of previous practice using Boil-Off Gas as fuel, the Re-liquefaction system establishes a solution to liquefy the Boil-Off Gas and return the LNG back to the cargo tanks. This Re-liquefaction of Boil-Off Gases on LNG carriers results in increased cargo deliveries and allows owners and operators to choose the most optimum propulsion system.
The design of the LNG Re-liquefaction plant has been performed based on the nominal BOR of 0.15 % of cargo capacity per day for the GTT and Moss insulation systems LNG carriers.
The Re-liquefaction system is basically made of two parts which is BOG cycle and Nitrogen cycle.
The Re-liquefaction process is carried out by condensation of BOG at a slightly elevated pressure of 4.5 bar against nitrogen gas which is to be cooled into a three-stage Brayton cycle.
BOG is removed from the cargo tanks by means of a two stage centrifugal compressor, which is similar to conventional LD Compressors. The BOG is cooled and condensed to LNG in a Cryogenic heat exchanger (cold box). Non-condensable items, mainly nitrogen, are removed in a separator vessel. From the separator, the LNG is returned to the cargo tanks by the differential pressure in the system. The cryogenic temperature inside the Cold Box is produced by means of a nitrogen compression-expansion cycle.
In this study, thermodynamic cycle analysis has been performed based on two type of LNG Re-liquefaction system which was designed and adopted for the Q-Flex(220,000㎥) and Q-Max(266,000㎥) LNG carrier under construction at Korea ship yards and variable key factor was simulated to compare COP, power and nitrogen consumption of each Re-liquefaction system cycle.
According to the result of this study, there is no notable difference in respect of COP and performance of Re-liquefaction system at the view point of thermodynamic cycle, moreover design and operation is to be considered to avoid liquid formation at BOG Compressor in case of installation of Intercooler between 2^(nd) stage Compressor.
For the development of high performance Re-liquefaction plant, it is essential to develop high efficiency compressor and turbo Expander and high performance heat exchanger is important factor to reduce power consumption and to increase COP and also effort to reduce cooling water temperature to be considered for design and operation of Re-liquefaction plant.