The debate on the need of the use of renewable sources seems to be very heated. Often, it happens that it falls into easy populism, without any technical /economic reason, and we do not realize the importance of a resource such as natural gas in the transition to a fully renewable energy system.
Natural gas is a fossil nature resource, containing mostly CH4 molecules, Methane, the simpler and more abundant hydrocarbon in nature, a number of other hydrocarbons in smaller quantities (ethane, pentane, butane ) and other impurities, removed during the process of gas extraction. The natural gas is the cleanest hydrocarbon during combustion. It is plentiful (referring on present rates of consumption) and versatile, very useful in coupling with renewable energy plants. It is transported through pipelines when overland, or with tankers by sea, after undergoing a liquefaction process, which allowsto obtain the LNG (liquefied natural gas). This last procedure seems to attract many investments.
The LNG supply chain can be divided into four phases: production, liquefaction, transportation and regasification. Production would deserve an in-depth analysis, because it affects the drilling phase, a much criticized practice, although no strong arguments have been offered in this regard. Remember that, we are talking about extracting gas, not oil, and the environmental impacts of extraction platforms are quite different. In Italy, we have the Adriatic LNG terminal, fueled by the largest gas field in the world, not associated with oil, located off the coast of Qatar, called North Field. NG at atmospheric pressure and temperatures is in a gaseous state, and for the terrestrial transport it is adopted a pipeline network. This procedure, though convenient and cheaper in the long term, has disadvantages: gas has to pass for some compression stations, in order to maintain a good transportation speed, with consequent expenditure of energy, and the pipeline network does not grant a capillary cover of all territories. The idea is to proceed to its cooling to -162°C, resulting in liquefaction, reducing its specific volume by 600 times. Then, every kg of gas, will occupy a volume 600 times lower than that which would have in the conditions of pressure and atmospheric temperature. Liquefaction plants behave as normal cryogenic refrigeration systems and are organized with more units in parallel, called trains. Each unit processes a gas portion. In the liquefaction trains, the pure gas is first treated to remove water, propane, heavier hydrocarbons and all of those impurities that may create malfunction, or solidify at low temperatures. The LNG is then pumped into storage tanks before being loaded on special tankers.
The LNG travels at a constant temperature and atmospheric pressure in special tankers, designed and built according to strict safety standards. The critical step is to maintain at appropriate levels its temperature during transportation, to ensure that the gas remains in the liquid state. This means that it is necessary to reduce, as much as possible, the thermal exchanges with the external environment. This result is obtained not only adopting appropriate materials and insulation for tanks, but also realizing them with a spherical shape, since it is well known that with equal volume the sphere is the solid that presents the lower surface area. The LNG industry has linked the gas fields scattered around the world, often far away and difficult to reach, to countries with the need of new sources of supply. The transport of LNG by ship makes available these energy resources in places that otherwise, would be almost unattainable, without facing significant economic and infrastructural efforts. In Italy, Sardinia case is a good example. The region is cut off from the network of natural gas. However, the works for the construction of a filling station of LNG in Oristano harbor started. The project includes six cryogenic tanks with total capacity of 9000 cubic meters. I t will be fueled by LNG carriers, creating an important reference point in the Mediterranean sea. The LNG will serve industries, civil users, but the aim is to use it in heavy vehicle transport and marine carriers. The key element of this phase is the re-gasification terminal. When LNG carriers come to the reference terminal, the liquefied natural gas is discharged from the ships and stored, always at a temperature of -162°C and at atmospheric pressure, in special tanks. After its heating in a controlled manner, the LNG returns to the gaseous state. This heating phase can be realized in different ways, depending on the technological level of the terminal. Heat exchangers can use marine water at environmental temperature, or in some cases, like Adriatic LNG, waste heat can be recovered from turbines, to heat the natural gas. At the end of this process that determines a natural expansion, the gas is fed into the national grid through a pipeline.
Italy is the third country in Europe for consumption of natural gas, but its local production covers only 10% of total needs. In the face of higher costs, some advantages of the LNG chain are significant, also in terms of safety. Its features make it hardly inflammable, becoming fuel only when it mixes with the air in a percentage between 5% and 15% and, in any case, not ignites spontaneously, but only in the presence of an external heat source. It is not toxic and it does not cause explosions. Even coming in contact with fire, the LNG does not create flare-ups, but only a lazy flame that evaporates without creating shock waves.
Recently, the European Commission considered necessary to issue a document that point out the liquefied natural gas as a strategic resource (Communication from the commission to the European Parliament, the council, the European economic and social committee and the committee of the regions, on an EU strategy for liquefied natural gas and gas storage). Its use seems to be affecting the heavy transport sector. The implementation of the project Maganetti group in cooperation with Iveco is recent and, involves the use of the Iveco Stralis LNG vehicle, with a totally LNG engine, equipped with cryogenic devices. As in most of the energy transitions, the results may not be immediately satisfying, but the experts seem to be convinced of the convenience of this switch. The same Maganetti group declares the disadvantages of the project, at the current state of art:
- Starting from the same equipment, there is a 20% higher purchase cost compared to a diesel vehicle;
- Lower power from 20% to 45 %;
- Reduced autonomy, 650 km compared to 1500 km of a diesel vehicle with medium tank.
However, if we care of sustainability, the benefits seem to be interesting:
- low taxation of natural gas compared to diesel, and reduced circulation costs ;
- reduced environmental impact, -40 % CO2 , NOx -50 % , -95 % particulate than EURO VI standards.
In conclusion, it might be necessary to rationalize the debate on renewable energy, realizing that, in order to be sustainable an overview of the energy system is required, with technologies that complement each other. Natural gas, although belonging to fossil fuels, currently, is the lesser evil, some forecasts say that the global LNG demand is projected to reach 500 million tonnes per year by 2030, an increase of 200 % compared to 2005 (Edison source). It is also the resource that will allow us to amortize, in the coming years, the technology gap in the renewable field, which currently does not allow us to maintain the welfare, to whom deep down, quite rightly, we are not willing to give up.
Written by Carlo Maccioni, MSc Graduating in Energy Engineering