Can Cargo Ships Run on Liquefied Gas?

For over half a century, cargo ships have been utilizing natural gas, albeit for different purposes compared to today's widespread use of liquefied natural gas (LNG) in modern merchant vessels.

Historical Context and Evolution

In 1965, natural gas was already being used in marine steam boilers to drive steam turbines. However, the application has evolved significantly. Today, 2-stroke MEGI and XDF engines, as well as 4-stroke Wartsila 50DF engines, are widely used in many merchant ships for propulsion.

Advancements in LNG Propulsion

On LNG carriers, which specifically use LNG, the 4-stroke Wartsila 50DF engines are part of the diesel-electric propulsion layout. These engines operate on the lean burn Otto process, which means the air and gas mixture in the cylinder has more air than necessary for complete combustion. This reduces peak temperatures and lowers NOX emissions.

Engine Operation Details

The fuel gas is introduced into the charge air manifold for the specific cylinder head through a solenoid-operated gas valve, up to 5 bar of pressure. Due to the higher self-igniting temperature of natural gas, which cannot be achieved by normal compression in the engine, a small amount of diesel oil is injected to serve as a precursor to the gas burning by increasing the compression temperature.

Power Output Variations

The power output available from such engines depends on the number of cylinders in the engine layout. One cylinder provides approximately 914 kW of power. Engines are available in various configurations, such as 68, 9, 12, 16, and 18-cylinder layouts.

Modern LNG Use in Shipping

The shift towards LNG as a marine fuel for cargo ships is gaining momentum. Early commercial vessels to adopt LNG as marine fuel, apart from LNG carriers, were the American TOTE Maritime company container ships equipped with MAN BW MEGI engines, which came into use in 2015.

Another pioneering vessel was the Norwegian ferry MV Glutra, which began its operations in 2000, making it the first modern LNG fueled vessel outside the LNG carrier fleet.

Advantages of LNG in Shipping

LNG is particularly favored in the shipping industry for several reasons. Firstly, LNG carriers can utilize the boil-off from their cargo to run the ship's engines, thus eliminating the need to vent boil-off to the atmosphere. This not only reduces operational waste but also improves environmental sustainability.

Furthermore, LNG engines are highly efficient. The vapor from LNG is added to the inlet air, and a small injection of diesel fuel is used to ignite the dual-fuel mixture. This process results in extremely clean burning, with minimal emissions and higher efficiency compared to traditional fossil fuels.

Conclusion: The transition to liquefied gas as a maritime fuel is transforming the shipping industry. From historical usage to modern advancements, the shift towards LNG offers significant benefits in terms of efficiency, sustainability, and compliance with environmental regulations.