BOG Management – Recycool

As environmental regulations evolve, there is a growing need to improve the energy efficiency and reduce the emissions of LNG-fueled ships. High-pressure MEGI engines, which have low methane slip and high power efficiency, are often selected for vessel main propulsion. However, with such high-pressure engines, finding a reliable, simple and cost-effective solution for boil-off gas management remains quite complex.

The fuel gas supply system of the MEGI engine requires a high-pressure compressor to manage boil-off gas of the LNG tank. However, ship-owners have preferred to install low-pressure compressors due to their lower cost. Thus, the boil-off gas can only be consumed by electrical generator, which limits the ability to manage the pressure of the LNG Tank.

The RECYCOOL^TM concept was developed by GTT. It allows to manage the excess boil off gas by recovering the cold energy available from the LNG vaporizer. Such cold power would otherwise be wasted into glycol or freshwater system.

The principle of the system consists of recovering the available cold energy in order to re-liquefy the excess boil-off. Cold recovery is performed in two locations: in a recondenser before the high-pressure pump and in a pre-cooler located between the high-pressure pump and the high-pressure vaporizer.

Compared to other existing recondenser solutions, Recycool^TM shows its advantage in terms of compactness and efficiency.

Note 1: Performance is independent of BOG temperature, since cold recovery is only performed on the LNG line – which allows maintaining high efficiency in a large working range

Note 2: Recycool high performance is increased by avoiding direct contact between the LNG stream and the BOG stream.

This concept has been already successfully tested on several LNG-fueled container ships powered by a MEGI engine.