When tankers transport liquefied natural gas (LNG), a slight evaporation of the cargo – known as boil-off gas (BOG) – is inevitable. To maintain the right tank pressure, it is essential to remove BOG from the tanks. The conventional method has been to use some BOG as fuel for propulsion, while excessive gas is burnt in a gas combustion unit. This not only releases harmful gases to the environment, but also treats most of the BOG as waste.
A far better alternative is to re-liquefy the excess BOG and make use of it. With Wärtsilä’s LNG BOG reliquefaction plants, surplus BOG is returned to the tanks and can be sold as cargo instead of going up in smoke.
The systems are efficient, reliable, safe, robust and flexible – and we offer solutions for all sizes of LNG carriers and bunkering vessels – as well as the retrofit market. With more than 50 systems in operation, Wärtsilä has the highest number of BOG reliquefaction plants installed in the global LNG carrier fleet.
Wärtsilä has been a pioneer in this field since the early 2000s, and is today a leading designer, developer and supplier of energy efficient LNG BOG reliquefaction plants. We supply complete and integrated systems including BOG reliquefaction and fuel gas supply systems as compressor solutions, and LNG pump solutions required for feeding engines with low and/or high-pressure gas.
Our BOG reliquefaction process is based on reversed nitrogen Brayton cycle refrigeration technology, using commercial grade nitrogen to extract heat from the boil-off gas. During this reliquefaction, the cargo is kept cool and at the right pressure under all operating conditions. The systems allows a portion of the BOG to be used as fuel for the ship’s engines, while the excessive liquefied gas is returned to the main tanks to be sold as cargo. The systems can be designed to handle all boil-off gas, or only the excessive gas not used by the engines (partly liquefaction).
Wärtsilä’s Compact Reliq module is our updated offering, translated into a compact and reliable module. The Compact Reliq is also instrumented for remote monitoring and online operational support as part of Wärtsilä’s OPERIM programme.
For smaller BOG capacities we also have a Mixed Refrigerant (MR) system which offers supreme efficiency.
Figure 1: Nitrogen loop
Figure 2: Compact Reliq
Figure 3: Mixed refrigerant (MR) loop (for onshore applications only)
Our BOG reliquefaction plants can be equipped on all sizes of LNG carriers, bunkering vessels
offshore installations and onshore facilities such as LNG terminals. The systems are equally suitable for both newbuilds and retrofits.
After commercialising and improving reliquefaction technology since the late 1990s, our BOG reliquefaction systems have been installed on more than 50 vessels. These include the entire Q-Flex fleet, counting more than 31 million cubic meters of LNG carriers driven by slow speed diesel propulsion engines. We have also equipped several LNG carriers with DFDE, XDF and MEGI propulsion systems with the same type of systems.
Onboard reliquefaction makes the LNG value chain greener, stronger, more efficient, and provides it with greater flexibility.
In addition to preventing harmful emissions from entering the atmosphere, Wärtsilä’s BOG reliquefaction systems allow ship and cargo owners to utilize BOG to lower vessels’ operating costs and increase the revenue by delivering more cargo. Another attraction is that the LNG tanks are cold enough to start loading when arriving at the terminal. This increases the utilisation of the ship – another cost-saving benefit.
With a growing environmental focus also among charterers, and a trend towards more short term and spot market driven contracts, having onboard BOG reliquefaction makes a vessel more attractive and flexible in today’s market.
Choosing Wärtsilä as your supplier, means getting a reliable and proven plant. We have the industry’s most extensive BOG reliquefaction reference list. Our track record is backed by the most comprehensive system solutions throughout the entire gas value chain – offering two different technologies, capability to supply larger and complex systems, competitive capex in all capacity ranges, optimized processes for low opex cost, a worldwide service network and smart digital solutions for optimal performance and 24/7 service.
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