LNG
What is LNG?
Liquefied natural gas (LNG) is methane gas converted into liquid form for easier storage and transportation. LNG occupies only about 1/600th of the volume of natural gas in its gaseous state. It is odorless, colorless, non-toxic, and non-corrosive, but flammable, freezing, and poses a risk of asphyxiation.
Initially, the gas is extracted and transported to a processing plant, where impurities like water, oil, mud, and other gases such as CO2 and H2S are removed to purify it. Mercury is also typically eliminated to prevent amalgamation with aluminum in the cryogenic heat exchangers. The gas undergoes a cooling process until it becomes liquefied and is then stored in tanks for loading and shipping.
The liquefaction process involves removing certain components like dust, acid gases, helium, water, and heavy hydrocarbons to avoid downstream issues. The natural gas is condensed into a liquid near atmospheric pressure (maximum transport pressure set at around 25 kPa/3.6 psi) by cooling it to approximately −162 °C (−260 °F).
LNG provides a much greater reduction in volume compared to compressed natural gas (CNG), resulting in a higher energy density. It is approximately 2.4 times denser than CNG and about 60% of the density of diesel fuel. This characteristic makes LNG economically viable for long-distance transportation in areas without pipelines. Specialized cryogenic sea vessels (LNG carriers) or cryogenic road tankers are used for its transportation.
Mainly, LNG is used for transporting natural gas to markets, where it is converted back into its gaseous state (regasified) and distributed through pipelines. While it can be used in natural gas vehicles, compressed natural gas is more commonly used. The relatively high production cost of LNG and the need for expensive cryogenic storage tanks have limited its widespread commercial use.
The heating value of LNG varies depending on the gas source and the liquefaction process used. Its higher heating value is approximately 24 MJ/L, while the lower heating value is around 21 MJ/L or 563,623 BTU/ft3. To compare different fuels, the heating value, also known as energy density, is expressed in MJ/L or gasoline gallon equivalent in BTU/ft3. LNG’s energy density is 2.4 times higher than that of CNG, making it cost-effective to transport natural gas by ship. While LNG’s energy density is similar to propane and ethanol, it is only 60% of diesel’s and 70% of gasoline’s energy density.
