The third part of “The new frontier of natural gas”a three-part study dedicated to natural gas technologies.
The world gas market has received a great and innovative boost thanks to the new technologies developed for the production, liquefaction, transport, regasification and marketing of natural gas.
The complete process involves the natural gas, once extracted from the deposit and brought to the surface, undergoes a treatment to eliminate water, nitrogen, carbon dioxide and other impurities. After this purification from foreign substances, it is pumped, through underground or underwater pipelines, to the liquefaction terminals. These are traditionally found on the seashore, near docks for ship transport, but the first entirely floating FLNG plants are already in production, i.e. plants placed directly in the sea and dedicated to the purification, treatment and storage of natural gas liquefied.
There liquefaction it is the first key process: the gas is progressively cooled to below -161.4 °C and undergoes a change in physical state, passing into liquid form. This allows 620-630 m to be compacted into a single cubic meter3 of natural gas. This change of state occurs in liquefaction plants that include pre-cooling, extraction and fractionation sections of heavy component residues. Once liquefied, the gas is stored in large tanks equipped with an efficient thermal insulation system that allows the stored gas to be kept below its evaporation temperature for an indefinite time.
The storage it is the process of storing natural gas in liquid form, at a temperature ranging from approximately -160°C at one atmosphere, to -110°C at 20 atmospheres. By reducing the volume of gas it becomes easier to transport and, indeed, store it.
The tanks used are insulated and keep the LNG below its liquefaction temperature. They can have different sizes and capacities depending on different storage needs. The spherical and largest possible cryogenic ones are economically more advantageous because they minimize the heat exchange surface compared to the volume of the tank itself.
The LNG is then pumped on board special methane tankers equipped with tanks similar to the previous ones and with cryogenic systems that allow the necessary thermal insulation to be maintained throughout the subsequent journey.
Once the methane tanker has reached its destination, the gas, always liquefied, is transferred from the ship to a storage tank inside the regasifier, where it maintains the same physical transport conditions. The latter, the second key element of the technology, is an industrial plant that allows the product to be brought back from the liquid state used in maritime transport to the gaseous state useful for land transport and final consumption. Regasification plants can be built on land (on onshore structures), or on the high seas (offshore), or on particular floating terminals anchored to the seabed called “floating storage and regasification units”. (Floating Storage and Regasification Unit, FSRU), like the Offshore LNG Toscana plant in front of Livornoactivated in 2013 or by Golar Tundrarecently docked at the port of Piombinonear Livorno.
The regasification process of liquefied natural gas
There are also real artificial islands (GBS, Gravity Based Structure), such as the Adriatic LNG Terminal off the coast of Porto Viroin the province of Rovigo, active since 2009.
Onshore plants are normally built in port facilities, in order to benefit from the necessary technical and logistical support. For safety reasons they are often excluded from the port basin, and are found on piers in the open sea. In Italy, the first structure ever built is precisely of this type – designed and built between the end of the 1960s and active since the beginning of the 1970s – and is located in Panigaglia in the province of La Spezia.
Inside the regasifier, the LNG is sent to a vaporizer which, by increasing its temperature, causes the reverse change of state with the expansion of the gas, which returns to its natural physical state. The temperature variation generally occurs through the heat exchange in tube bundles between liquid gas and sea water, which transfers its heat to the gas; the pressure is instead reduced through the expansion of the gas in special tanks. At this point the gas can be introduced into the national distribution network.
The regasifiers can be combined with systems that require the use of low temperatures (for example food freezing plants), thus recycling the refrigeration energy with evident energy savings. Furthermore, the cold water coming out of 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 waters. In Italy, the Department of Energy and Environmental Research at the University of Palermo is studying these possible applications.
