LNG Temperature: Why −162 °C?

What is the temperature of LNG at atmospheric pressure?

At atmospheric pressure, LNG sits at roughly −162 °C. Pure methane boils at −161.5 °C (111.7 K) at one bar; because commercial LNG also contains ethane, propane, butane and a little nitrogen, real cargoes have a bubble point somewhere in the −158 °C to −164 °C band depending on composition. Leaner, methane-rich LNG from Qatar sits nearer the cold end of that range; richer LNG carrying more ethane and propane sits slightly warmer.

The phrase “at atmospheric pressure” matters, because temperature and pressure are not independent. Raise the pressure and methane will stay liquid at a warmer temperature; that is the principle behind pressurised small-scale LNG tanks and, in a different form, behind compressed natural gas. Large-scale LNG deliberately does the opposite. It accepts a very cold liquid so that tanks and ships can be built thin-walled and enormous rather than thick-walled and small.

What temperature turns natural gas into a liquid?

There are only two ways to condense a gas: cool it, or compress it. For methane, compression alone does not work at ambient conditions. Methane has a critical temperature of −82.6 °C, and above a substance’s critical temperature no amount of pressure will produce a liquid. Squeeze methane at room temperature and it simply becomes a very dense gas.

So the gas has to be chilled below −82.6 °C before liquefaction is possible at all, and in practice it is taken all the way down to the atmospheric boiling point so that the finished product can be stored in ordinary low-pressure tanks. That is the temperature the whole LNG chain is built around.

The payoff is density. One cubic metre of LNG holds the energy of roughly 600 cubic metres of natural gas. Without that compression of energy into volume, moving gas across an ocean would never pay — which is the argument set out in LNG vs. pipeline gas.

How is LNG cooled down and kept at −162 °C?

Cooling and staying cold are two different problems, solved in two different places.

Getting there: the liquefaction plant

Cooling happens once, on shore, in a liquefaction train. Treated gas passes through cryogenic heat exchangers against a circulating refrigerant — propane and a mixed refrigerant in the widely used C3MR process, or a cascade of pure refrigerants in other designs. The refrigerant is compressed, cooled, then expanded, and the expansion is what pulls heat out of the gas stream. Doing this consumes something like 8–10% of the energy in the feed gas. The full sequence is described in how LNG is made from natural gas.

Staying there: autorefrigeration

After that, nothing actively refrigerates the cargo. An LNG tank is not a freezer. Tanks and ship holds are insulated vacuum or membrane structures, and they rely on a physical trick called autorefrigeration: heat that does leak in is spent boiling a small fraction of the liquid rather than warming the bulk of it. The evaporating vapour carries the heat away, and what remains stays at its boiling point.

The vapour produced this way is boil-off gas. On a modern carrier, 0.1–0.15% of the cargo boils off per day; older ships lose 0.2–0.3%. That gas is not vented. It is burned in the ship’s engines as fuel, or reliquefied on vessels fitted to do so — the arrangements are covered under LNG carriers. The consequence is that a cargo stays cold for weeks at sea without any refrigeration plant running, at the cost of slowly getting smaller.

What −162 °C does to materials

Cryogenic temperature dictates what the industry can build things out of. Ordinary carbon steel becomes brittle when chilled this far and can fracture without warning, so it cannot touch LNG. Tanks, piping and heat exchangers are built instead from 9% nickel steel, austenitic stainless steel, aluminium alloys or Invar — materials that stay ductile in the cold and, in Invar’s case, barely contract at all.

Thermal contraction is the second constraint. A long run of stainless pipe shrinks measurably on cooldown, so cryogenic systems are fitted with expansion loops and bellows, and plants are cooled down slowly and deliberately to avoid thermal shock. It is also why a spill is dangerous to structures beneath it: LNG landing on an unprotected steel deck can embrittle it in seconds. Those consequences are dealt with under LNG safety.

Two common misconceptions

“LNG is under high pressure.” It is not. Large-scale LNG storage runs at a fraction of a bar above atmospheric. Cold does the work, not pressure. This is the single clearest difference between LNG and CNG, which is stored as a gas at 200–250 bar and ambient temperature.

“The cold liquid burns.” Liquid LNG does not burn. Only the vapour does, and only when it has mixed with air to between roughly 5% and 15% methane by volume. Too little vapour or too much, and there is nothing to ignite. A spill is hazardous mainly through cryogenic burns, material embrittlement and the flammable vapour cloud that forms as it warms.