If natural gas is so useful, why does it have to be liquefied? What's the point in converting this hydrocarbon from its natural state into a liquid form and then back again? And how exactly does a liquefaction plant act like a 'giant refrigerator'? BP Magazine looks at the science behind the process that allows gas molecules to go global
“You don’t find LNG, you make it,” says Mike Winstanley, talking about liquefied natural gas. BP’s head of engineering for the supply and trading business is responsible for managing BP’s technical involvement in many of the terminals that support its LNG trading activity. BP is also a partner or operator in six LNG export plants around the world used to convert natural gas in its gaseous state into a liquid that is much easier and more convenient to store and transport. BP’s figures show that the world’s demand for LNG is growing, helping to meet the total global demand for energy, which will be about a third larger than today by 2035. Natural gas, from which LNG is made, has a big role to play in the future, given its abundance, affordability and lower-carbon content in comparison to coal. The technology behind LNG is enabling natural gas to make an increasingly important contribution to the global energy mix by connecting gas in remote locations, such as BP’s Tangguh plant in Indonesia, with markets too distant to be connected by pipeline. “You just can’t build a pipeline from Africa to South America,” says Winstanley, by way of example. But, with gas in its liquefied form, transportation by ship - or sometimes road - becomes possible.
How is natural gas liquefied?
There are a series of processing stages to produce LNG. The first stage is to remove impurities - the level and type of impurities will depend on the composition of the gas from the upstream production. The natural gas then passes through a water-based solvent that absorbs carbon dioxide and hydrogen sulphide, which would otherwise freeze and cause blockages in the pipes when the gas is cooled. Water is also removed by passing the gas through a molecular sieve (desiccant) bed, as this too would freeze.
The last stage of purification involves removing any traces of mercury. The main components of the natural gas, methane and ethane, are then separated from the heavier components in a pre-chilling stage. The hydrocarbons are separated by fractionation, a process of separating components with different boiling points, and the heavier components either stored for use as fuel in the plant or sold as by-products. What’s left is the purified natural gas - methane, ethane and small amounts of propane and butane - which is now ready to be liquefied. The liquefaction plant acts like a giant refrigerator, chilling the gas until it reaches minus 162 degrees centigrade at atmospheric pressure, turning it into a liquid that occupies 1/600th of its volume as a gas.
What happens next?
The LNG is stored in large insulated storage tanks, which typically hold 160,000 to 200,000 cubic metres. The LNG is then pumped into specially-designed LNG carriers, such as BP Shipping’s Gem-class vessels that can carry a cargo of 155,000 cubic metres, equivalent in volume to 62 Olympic swimming pools.
A small amount of LNG naturally vaporizes during the voyage and this ‘boil off’ is either captured and recycled or used as fuel supplies powering the tanker. On arrival at the cargo’s destination, LNG is transferred into storage tanks at dedicated import terminals, where it is kept as liquid until needed.
“The regasification process is a lot simpler,” says Winstanley. “All you’re really doing is heating up the LNG until it vaporizes.” This is typically done with seawater, which is much warmer than the LNG, or by passing the LNG through a heated water bath.
However, the purification done during LNG production has an impact at this end. “Sometimes, it’s almost a bit too clean because so many of the impurities and heavier hydrocarbons have been removed.”
BP produced in excess of 10 million tonnes of LNG in 2015, around 4% of the global market.
To counter this, LNG may need to be blended with an inert gas, such as nitrogen, or have propane added to make it compatible with the composition of the natural gas in the local network. This can differ from country to country, so the dilution formula is specific to each individual country or network.
Once regasified, the LNG is transported in pipelines and used as any other natural gas, whether that’s fuel for power stations or direct supply into homes.