Technological innovations open up new scenarios in the gas market

The third in-depth analysis of 'The LNG Revolution', a unique three-part format dedicated to the evolution of natural gas, a key driver for the energy transition.

by Luca Longo

5 min read

by Luca Longo

5 min read

The technology

The revolution produced by the new 'world gas market' which now joins the already established oil market originates from the new technologies developed for the production, liquefaction, transport, regasification and, finally, marketing of Liquefied Natural Gas (LNG).

Once extracted, the natural gas undergoes treatment to remove water, nitrogen, carbon dioxide and other impurities. It is then pumped through underground or underwater pipelines to liquefaction terminals. These are traditionally found on the seashore, near docks for transport by ship, but the first fully FLNG floating plants which are similar to the FPSO used for oil but with all the necessary infrastructure for liquefaction and storage on board, are already in production.

Liquefaction and transportation

Liquefaction is the first key process. The gas is progressively cooled down to below -161.4 °C, at which point the gas undergoes a change of physical state and transforms into liquid form. This allows as much as 620-630 m³ of natural gas to be compacted into a single cubic metre. The process takes place in liquefaction plants consisting of one or more lines (trains) of equal capacity that operate simultaneously, each comprising the pre-cooling, extraction and fractionation sections of heavy component residues, liquefaction and storage in large tanks equipped with an efficient thermal insulation system that allows the stored gas to be kept below its evaporation temperature indefinitely.

When stocking, pressure resistance is not an issue as generally these tanks are tested to withstand pressures of a few tens of atmospheres (Bar). By comparison, those containing natural gas in a gaseous state and at ambient temperature, for example in vehicles, are designed for operating pressures of more than 200-300 Bar. The critical factor for LNG is efficient thermal insulation. Tanks are built to maintain a temperature and pressure range from around -160°C at 1 Bar to -110°C at 20 Bar. This is why it is economically advantageous for cryogenic tanks to be spherical and as large as possible, thus minimising the heat exchange surface area in relation to the tank volume.

The LNG is then pumped aboard special LNG carriers equipped with similar tanks and cryogenic systems to maintain the necessary thermal insulation throughout the subsequent journey.

Regasification

Once the LNG carrier has reached its destination, the gas which is still liquefied, is transferred from the ship to a storage tank inside the rigasifier, where it maintains the same physical conditions as during transportation. The latter, the second key element of the technology, is an industrial plant that allows the product to be returned from the liquid state kept during transportation at sea to the gaseous state, more suitable for transportation over land and for its final consumption. Regasification plants can be built on land (on onshore structures), or on the high seas (offshore), or on special floating terminals anchored to the seabed known as 'Floating Storage and Regasification Units' (FSRU), such as the Tuscany Offshore LNG plant off Livorno, which was activated in 2013 or the Golar Tundra recently located in the port of Piombino, in the neighborhood of Livorno.

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The natural gas liquefaction regasification infrastructure

There are also real artificial islands (GBS, Gravity Based Structure), such as the Adriatic LNG Terminal off Porto Viro in the province of Rovigo, which has been active since 2009.

Onshore plants are normally built at port facilities in order to benefit from the necessary technical and logistical support. For safety reasons, they are often excluded from the harbour basin, and are located on jetties in the open sea. In Italy, the first structure of this kind which was designed and built in the late 1960s and has been active since the early 1970s, is located in Panigaglia in the province of La Spezia.

Inside the regasifier, the LNG is sent to a vaporiser which, by raising its temperature, causes a reverse state change as the gas expands and returns to its natural physical state. The temperature change usually occurs through heat exchange between liquid gas and seawater in shell-and-tube heat exchangers, which gives up its heat to the gas; the pressure, on the other hand, is reduced through the expansion of the gas in special tanks. At this point, the gas can be fed into the national distribution network.

Regasifiers can be combined with plants using low temperatures (e.g. food freezing plants), thus recycling cooling energy with obvious energy savings. Furthermore, cold water leaving a regasifier can be exploited as a cold source in an energy production plant that exploits the thermodynamic gradient with respect to the temperature of the surrounding water. In Italy, the Department of Energy and Environmental Research at the University of Palermo is studying these possible applications.