DieselShip

LNG reliquifaction

Traditionally, LNG carriers have used the boil off from cargo as fuel for the propulsion. This led to the use of steam plant as the preferred method as the combustion of this gas is simpler. In addition the steam plant gives the option to safely deal wih excess boil off of gas during pasage or arrival in port without the need to resort to venting.

The use of duel fuel burning diesel engines is becoming more in favour thus requiring alternative methods to deal with the excess boil off.

One method is reliquifaction commonlu seen in LPG shps but previously not sen in LNG due to the very low saturation temperatures of LNG

Reliquifaction process

This generally takes the form of seperate reliquifaction and refrigeration units. The system uses Nitrogen as the refrigerant in the reverse Brayton cycle; a version of the cascae system in which several stages ( normally 4) operate in series with the LNG. The final stage is a turbo expander in which the energy of the Nitrogen is used to power the expander stage. Thus with the removal of energy from the nitrogen in the form of work the temperature is low enough to liquify the LNG.

The entire asembly my be either totally enclosed in insulation with repairs only to be carried out at predetermined times. Or it may be enclosed in an insulated work house

It should be noted that hgh levels of nitrogen in the cargo can have a serious effect on the viability of reliquifaction

The power absorbed by this unit is high thus ony recent increases in cargo prices has made it economically viable. A high voltage distribution system is required.