LNG is the name given to the liquid form of natural gas, obtained by lowering the temperature to a value of -160°C. Liquefied natural gas is transparent, odorless, non-corrosive and non-toxic.
The main advantage is that for the same amount of gas, the volume occupied by LNG is 600 times less than the volume of the same amount in gaseous form.
In this article, discover the characteristics of LNG and the main differences from LPG.
What is Liquefied Natural Gas (LNG)?
LNG, which stands for liquefied natural gas, is created with a process of gas cooling and condensation. The main component is methane whose percentage can vary between 90 and 99%, the remaining part is made up of butane, ethane and propane. LNG at temperatures above -112°C is lighter than air and disperses quickly, not contaminating the ground. LNG is liquefied in producing countries and then transported to its destination across the sea with special tankers.
Liquefied natural gas is an alternative fuel to be used in those areas not covered by the methane gas network. It is generally used where the need for gas is high, therefore for industrial users that use gas for their production process. In these cases, consumption is constant throughout the year, unlike domestic users where there is a seasonality of consumption, higher in winter due to heating. Small and medium-sized municipalities not reached by the gas network or entire islands can also be served with LNG.
LNG is a fuel with low environmental impact, with zero particulate emissions and low CO2 emissions. It is not corrosive and it is a clean gas because it does not release particulates, sulfur or sulfur compounds.
How is Liquefied Natural Gas Produced?
To make methane gas liquid, a not simple liquefaction process is required. The treatment involves purification and dehydration and subsequently alternating phases of compression and cooling of the methane.
During the condensation process (liquefaction) its volume is reduced by 600 times, allowing to store a considerable amount of energy in a small space. In simple terms this means that it takes 600 liters of methane gas to obtain one liter in liquid form.
Its density allows a large amount of energy to be stored in a small space. The calorific value of LNG, i.e. the amount of energy that a fuel releases, is higher than that of LPG and more significantly than diesel or BTZ (low sulfur fuel oil).
The gas is brought up to a temperature of about -160°C and in this suitable condition it can be stored in the liquid state in special containers.
The Liquefied Natural Gas Tank is Cryogenic
In addition to being introduced into the national network, LNG can also be stored in liquid form at the end consumer.
Considering the conditions necessary to keep it in the liquid state, particular cryogenic tanks are indispensable, similar to those for liquid nitrogen. In these containers it is possible to keep the LNG temperature inside at -160°C. LNG tanks can have horizontal and vertical axis with different storage capacities according to needs.
The cryogenic tank consists of two casings, one contained within the other. The external casing is made of carbon steel while the internal one is made of resilient material, resistant to low temperatures. In the interspace between the casings, the air is eliminated and a material such as perlite is inserted, which serves to increase insulation and decrease the transmission of heat inside.
The vaporizer placed near the tank, as you can see in the next image, causes the LNG to return to the gaseous phase using the atmospheric heat to be then used.
What are the Commonalities and Differences Between LNG and LPG?
LNG and LPG (Liquefied petroleum gas) allow users not served by the methane distribution network to exploit the gas for their energy needs. Both LNG and LPG are transported to the final customer by tankers and stored in tanks before being consumed. Both can be used for space heating or for the industrial process or for the production of steam. LPG is also distributed in lower capacity cylinders when consumption is relatively low.
The main differences between LNG and LPG are:
- Different composition: LNG is mainly made up of methane while LPG is made up of butane and propane
- LNG is liquid at a temperature of -160°C: the tank for storing LNG must be cryogenic, on the contrary, LPG can be stored at higher temperatures even in simple cylinders or in normal non-refrigerated tanks
- For these reasons, LNG is adopted with significant and regular consumption of gas, as for industrial process use, while LPG is also used for low and medium consumption such as the domestic ones of a family.
As mentioned previously, liquefied natural gas is a source of clean energy since with liquefaction pollutants such as carbon dioxide and hydrogen sulphide are eliminated. The carbon dioxide emissions from the combustion of LNG are therefore lower than most fossil fuels. The table shows how LNG has almost zero levels of fine dust.
What is LNG Cogeneration?
By cogeneration we mean the combined production of electricity and heat, starting from the same energy source, LNG. In a common power plant there is a high dissipation in the form of heat. The principle behind cogeneration is to try to recover the heat that would be lost and use it to maximize the efficiency of the plant. Thermal energy can be recovered by producing steam or hot water, which can be used directly at the user.
An LNG cogeneration plant is installed when there is a simultaneous need for electricity and heat.
In general, most cogeneration applications are for industrial users, however they are also used in the civil sector with district heating. Combined production makes it possible to obtain a better efficiency with benefits in terms of fuel and therefore economic savings and reduction of polluting emissions.
Why Cogeneration Takes Place
When electricity is produced, the greatest waste is in the loss of residual heat to the external environment. The best performing electricity generation plants achieve an efficiency of 55%. With cogeneration we try to improve this efficiency by exploiting this heat that would otherwise be lost. The joint production of electricity and heat makes it possible to achieve a higher overall efficiency, over 80%.
This type of systems are therefore suitable when there is a demand for electricity and heat at the same time, for example in a factory that needs electricity and hot water or steam. Heat can also be used to meet the demand for multiple homes (district heating).
Advantages and Disadvantages of Cogeneration
However, it should be remembered that as the technology used to carry out cogeneration changes, the performance of the system changes and savings are not always taken for granted. The technologies are varied and on these depends the power size of the cogeneration plant, which varies from a few kW up to hundreds of kW. In general, cogeneration plants consist of an engine, such as a turbine (gas or steam) or an internal combustion engine, an electric generator and a heat recovery system assembled together.
The engine has the task of converting the fuel into mechanical energy which is then transformed into electrical energy by the generator, while the heat recovery takes place via an exchanger, that is a recovery boiler, which collects the exhaust fumes before discharging them into the atmosphere.
Let’s see briefly what are the disadvantages of cogeneration:
- Congruence of the temperature of the heat produced with the actual request of the same by the user
- Plant location: close to the heat users (distribution costs and high losses for large distances)
- Time lag of power and heat demands (obvious heat storage difficulties)
On the other hand, the advantages are many:
- Economic savings due to lower fuel consumption
- Reduction of the environmental impact
- Lower transmission and distribution losses for electricity
- Replacement of less efficient and more polluting systems (such as boilers, both civil and industrial).