LNG as a fuel

Although the marine industry relies heavily on fuel oils for the majority of its power generation, stricter emission regulations and the plan to “go green” are convincing operators to consider alternatives. To comply with the new emissions standards known as IMO 2020, a growing number of the world’s shipping companies, builders, and operators are turning to LNG as a cleaner alternative fuel source. The LNG is safe, reasonably priced, plentiful, emits small amount of the sulfur oxide, reduces carbon emission and other fuel pollutants and is generally regarded as a source of transition throughout the world. In comparison to heavy fuel oil (HFO), LNG enables significant reductions of more than 99% of sulphur emissions, 80% of nitrogen oxides, the majority of particulate matter, and up to 20% of carbon dioxide emissions.

Thus, LNG is regarded as the greenest fossil fuel and a true technological breakthrough in terms of meeting compliance while maintaining competitiveness.

Liquefied natural gas (LNG) is primarily composed of methane (CH4), the hydrocarbon fuel with the lowest carbon content and thus the greatest potential for CO2 emission reduction. However, methane is a potent greenhouse gas, and methane release must be controlled to ensure that greenhouse gases emissions are reduced when LNG is used.

LNG is produced through a cryogenic process in which natural gas is filtered and cooled to -1620 C, resulting in a volume reduction of up to 600 times. This process converts the gas to a liquid, making it easier to store and transport. In its liquid state, LNG will not ignite. As a result, LNG must be stored in insulated tanks for cryogenic applications, which are expensive in comparison to traditional petroleum-based fuel storage and supply systems.

Impurities, water, and other associated liquids are present in natural gas extracted from the ground.

Natural gas is first processed to remove impurities. It travels through a series of pipes and vessels, where gravity aids in the separation of the gas from some of the denser liquids. Additional impurities are then removed. After that the natural gas is injected into a water-based solvent, which absorbs carbon dioxide and hydrogen sulphide. Normally, these would freeze when the gas is cooled, resulting in blockages and therefore any remaining water is drained, as it will freeze.

Finally, the remaining lighter natural gas liquids – primarily propane and butane – are extracted for sale or use as a refrigerant during the cooling process. Mercury traces are also filtered out.

Now that the natural gas has been purified – methane with a trace of ethane – it is ready to be liquefied and this process takes place in large heat exchangers. A coolant, chilled by massive refrigerators, absorbs the natural gas’s heat. It cools the gas to -162°C, effectively halving its volume. This converts it to a clear, colorless, non-toxic liquid known as liquefied natural gas, or LNG, which is significantly easier to store and transport. The LNG is stored in insulated tanks until it is ready to be loaded onto an LNG ship or carrier designed specifically for the purpose.

Source: Shell – LNG process flow diagram

LBG (Bio-LNG) and liquefied synthetic methane (from the Power-To-Gas process) are fully compatible drop-in replacements for gas engines and the LNG distribution system. The material properties are very similar to those of LNG and can be considered identical in all practical ways.

LNG bunkering requires special infrastructure for supply, storage and delivery to the vessels. Presently there are not so many ports worldwide capable to offer LNG bunker as infrastructure is limited, but as the demand is increasing exponentially most of the major port are developing the necessary infrastructure.

The technology needed to use LNG as a ship fuel is readily available and MAN Diesel and Wartsila are already on the market with their dual fuel engines and half of all newbuildings after 2025 will be equipped with this type of engines.

A large number of vessels in the current ocean going fleet have the potential for conversion, but retrofitting existing ships seems to be less desirable on certain type of vessels, because of the impossibility for fitting special and larger fuel tanks.

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