What is LNG and how is it transported?

Liquefied Natural Gas (LNG) is an effective way to transport gas to areas where pipelines do not reach. The demand in LNG has kept growing, until it became a major energetic and economic stake. What is the process of making LNG, from liquefaction to redistribution?

To answer the demands of a growing population, economic development, energy consumption, and environmental pressure, liquefied natural gas (LNG) has grown as a major energy source expected to provide a quarter of the global energy demand in 2030. By 2035, natural gas could overtake coal as the second-largest fuel source of primary energy.

Natural gas is generally transported through pipelines, which are suitable for short- to medium-length distances. However, onshore pipelines longer than 4,800 km and offshore pipelines longer than 1,600 km are less economical than LNG(1).

What is LNG(2)?

Liquefied natural gas is mostly composed of methane (85-95%) but also contains a small percentage of ethane, propane, and butane (5-15%), as well as traces amounts of nitrogen. It is an odorless, non-toxic, non-corrosive liquid with low chances of inflammation.

Matter transitions from gaseous to liquid state under the right conditions of temperature and pressure: for example, at atmospheric pressure, liquid water enters its gaseous phase at 100 °C (212 °F). Under atmospheric pressure and ambient temperature, methane and all the other components of LNG are in a gaseous state. Liquid methane enters its gaseous phase at -162 °C (-260 °F), which is about the lowest temperature that occurs on the moon(3). To obtain LNG, natural gas must be cooled to its liquid state, a process called liquefaction.

The volume of natural gas in its liquid state is 600 times smaller than in its gaseous state: the same amount of natural gas that would fill a beach ball can fit in a golf ball when liquefied. This property makes its containment and shipping a lot easier.

Exhibit 1 - LNG global exports(3)

In 2021, Australia was the biggest producer of LNG with 87.6 million metric tons mainly exported to the Asian market. Qatar followed with 77.4 million metric tons, sent all over the world. Lastly, the US produced 71.6 million metric tons.

LNG exports account for 45% of the US natural gas export; almost half of this gas is sent to five main countries. The US is however expected to be the world’s largest LNG exporter in 2022, as a new export facility and an additional liquefaction unit will be completed at the end of the year.

Above is the breakdown of US LNG exports in 2020, with the five main importers that receive almost half of the resources. (Source: US Energy Information Administration)

The liquefaction process(1)

A purification step is performed before the liquefaction: dust, pollutants, CO2, and water are removed to avoid corrosion and freezing in the pipes. Natural gas liquefaction is based on a refrigeration cycle, a four-step process that aims to progressively absorb heat from a gas using a liquid called a refrigerant. By repeating the cycle, natural gas is cooled continuously until reaching its temperature of liquefaction.

1. First, the compressor increases the pressure of the refrigerant: it enters as a low-pressure, low-temperature gas and leaves as a high-pressure, high-temperature gas.
2. Then, the condenser removes heat from the hot refrigerant vapor coming out of the compressor until it condenses into a liquid. The refrigerant is then a high-pressure, low-temperature liquid.
3. The expander creates a drop in pressure after the refrigerant leaves the condenser. It becomes a low-pressure, low-temperature liquid.
4. Finally, the refrigerant enters the evaporator: this is the step where the cooling is provided. A fan forces natural gas to flow through the evaporator and its heat is absorbed by the refrigerant that progressively turns back into gas. The refrigerant is then sent back to the compressor where the process restarts.

Two main methods are used to cool natural gases: the cascade method has three refrigeration cycles that respectively use propane, ethylene, and methane as refrigerants. Each cycle brings natural gas to a lower temperature. The mixed refrigerants method (MR) is a single cycle that uses a blend of multiple light hydrocarbons as a refrigerant. Cascade is more energy-efficient but requires more space, which limits its use to large-scale onshore plants (with a capacity superior to 1 million tons of LNG p.a.). MR is used onshore and offshore for small- and large-scale plants; the hydrocarbon mixture can be adjusted to optimally follow the cooling curve of natural gas.

LNG export terminals consume some of the gas delivered to the facility to operate the liquefaction equipment. The US Energy Information Administration estimates that about 15% to 18% of the natural gas delivered to LNG export facilities is used for liquefaction(3).

To meet the increasing demand for natural gas, research institutes and companies are trying to develop small-scale LNG plants to exploit the abundant smaller-sized gas resources, as well as more offshore plants. However, offshore plants have different requirements than their onshore counterparts: the small footprint of the equipment, the ease of maintenance, and the sensitivity to motion are crucial because of the lack of deck space and the surrounding ocean environment.

LNG transportation(2)(3)

After liquefaction, LNG is stored in tanks at a pressure slightly above atmospheric pressure. Those tanks often have an inner shell made of a special alloy of stainless steel, aluminum, or nickel, designed to resist low temperatures, and an outer concrete shell with reinforced slab and roof. Each tank is insulated to maintain LNG at the right temperature and is equipped with sophisticated protection systems that monitor the liquid level, pressure, temperature, and detect potential leakages.

Transportation is provided by special tanker ships with a double hull to limit the risks of leaks and spills in case of collisions. For overland delivery, trucks are the preferred solution, especially in the US; trains can also be used as carriers but are often avoided in populated areas, as an accident involving more than 100 tanks could potentially destroy a city.

At import terminals, LNG is offloaded from the ships and stored before being turned back into gas to be transported through local pipelines. The regasification is done by heating LNG: a small amount can be burned, which produces the heat necessary to gasify the remaining liquid. Or, more frequently, LNG goes through a heat exchanger, a device that transfers heat from one medium to another; the heater used is ambient seawater or river water that is returned to its source afterwards.

Exhibit 2 - LNG carriers(4)

LNG ships (also called tankers, carriers, or vessels) are specifically equipped to transport LNG in bulk. Modern ships carry in average between 125,000 m3 and 175,000 m3 of LNG. They are divided into three main types. The Moss type carries spherical tanks that are self-supporting, meaning they are not a part of the ship hull and are not essential to its strength. Tanks sit half above the deck level, and half within the hull.

In the membrane type, LNG is maintained at low temperature by using thin inside tanks and covering them with a wrinkled stainless steel “membrane” (see: the inside of the tank). They are not self-supporting: the outer part of the tank is in contact with the hull that also helps to maintain insulation. The tank provides high visibility towards the front of the vessel because of the minimal protrusions on the deck. The biggest carrier, the Q-max, is a membrane type that can carry up to 266,000 m3 of LNG. An ice-breaking LNG membrane carrier has also been developed for Russia to extract gas in Yamal, an isolated region in the Arctic. It is equipped with an ice bow to break up to 2.1 m thick ice, as well as protections to prevent deck equipment from freezing.

The self-supporting prismatic shape IMO type B (SPB) carries an independent tank made of an aluminum alloy or stainless steel to provide heat insulation. It is broadly used because the tank can be built to match the hull shape, therefore increasing space efficiency; it also has no protruding structures on the deck. It is particularly adapted to serve in rough seas.

References

(1) J. Zhang, et al., Comprehensive review of current natural gas liquefaction processes on technical and economic performance, Applied Thermal Engineering, vol. 166, 114736, Feb. 2020 - (2) Cameron LNG - (3) US Energy Information Administration - (4) Mitsui O.S.K Lines

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