Natural gas is the last fossil source of energy to be used on a global scale. For over a century, when the gas was discovered in areas that were far from the places where it could be utilized, it was preferably burnt at the gas well or freed into the atmosphere, because harnessing it in a pipeline and making it travel for many kilometres was too expensive. The situation has changed over the past forty years and today natural gas ranks third in world consumption of energy and is the fossil source with the best growth prospects.
Natural gas is a fossil fuel like oil and coal. It is a mixture of hydrocarbons, mostly methane, and other gaseous substances such as carbon dioxide, nitrogen, hydrogen sulphide and, in some cases, helium. The mixtures that are mainly composed of methane are called dry mixtures, whereas those mainly containing hydrocarbons such as propane, and butane are called wet mixtures.
Before being distributed for use, natural gas is treated to eliminate carbon dioxide and nitrogen, which make it less flammable, and hydrogen sulphide, a corrosive and toxic gas. The result is mainly methane. Methane is the simplest gaseous hydrocarbon and is characterised by the smallest molecule, including one carbon atom and four hydrogen atoms (CH4). It is lighter than the air (at a 15°C temperature and 1013.25 millibar pressure its specific weight is 0.678 kg/m3), it is colourless and odourless and not toxic.
It was very common in the primordial atmosphere of the Earth and probably contributed to the synthesis of the first amino acids and the creation of life on our planet. Mixed with the air, methane becomes flammable only if its concentration ranges between 5 and 15%. Below 5% the quantity of natural gas is insufficient to start the combustion, whereas above 15% the oxygen is insufficient. At temperature of 15°C and atmospheric pressure 1 cubic metre of methane produces over 8 thousand calories. In those conditions one cubic metre of methane has an energy content equivalent to 1.2 kg of coal and 0.83 kg of oil.
Methane becomes liquid at a critical temperature of –83°C and a pressure of 45 bars. The transformation into the liquid state can take place by decreasing temperature or increasing pressure. For example, at –161°C methane becomes liquid at room pressure. The critical temperature of wet natural gases such as propane and butane, including 3 and 4 carbon atoms respectively, is higher than room temperature, therefore they become liquid simply by increasing pressure.
The origins of natural gas
On our planet hydrocarbons, including methane, are mainly located on rock pores that constitute the higher part of the Earth’s crust and result from chemical and physical processes that have taken place during the history of the Earth.
Where it is
The geographic location of gas reserves obviously mirrors that of oil: Russia, Iran and Qatar approximately have 54% of the total. Just like oil, the exploitation of gas fields takes place in a non-uniform way.
World natural gas reserves total 213,253 billion cubic metres approximately (data refer to 31st December 2020). Such reserves include the currently known fields that can be exploited through the technology available to obtain an economic profit.
A bit of history
In a manuscript by the Chinese historian Chang Qu dated 347 B.C. a strange gas is described that can be used for illumination purposes. Approximately 200 years ago, Alessandro Volta “re-discovered” the energy potential of natural gas noticing small gas bubbles which formed when the muddy bottom of the Maggiore lake was stirred.
Natural gas supplies are mainly used to generate electricity in many types of power plants where it fuels the burners that produce steam to drive turbines. It is also used in gas turbine power plants where, however, it is directly burnt in internal combustion turbines. Gas-fueled thermoelectric power stations are the best power plants because they combine a gas turbine (first stage) with a steam turbine (second stage) whose boiler is fueled by the flue gases leaving the gas turbine. These plants are known as combined cycle power plants. Natural gas is also now used for heating via district heating networks or in homes, businesses and industrial plants, for example for cooking, domestic hot water, and building heating or cooling systems.
The production of electric power
Thanks to its several economic and environmental benefits, in the latest years natural gas has converted into the main fossil fuel for the production of electricity. During the 70s and 80s, energy production was oriented towards coal and nuclear plants, but a series of economic, environmental and technological factors has provoked a shift towards the gas.
Among the innovative implementations of natural gas mention has to be made to cogeneration, i.e. the combined production of electric energy and heat. Cogeneration is the combined use of a primary energy, like natural gas, to produce heat and electricity.
Natural gas can also be used to fuel vehicles. Natural gas presents numerous advantages respect to other fuels for vehicles: the first of these is that it is an ecological fuel.
Industries make use of natural gas not only to heat or cool environments, but also to make production processes more efficient, cheaper and ecological.
The commercial use of gas includes cooling (conditioning and refrigeration), restaurant services (kitchen), motels and hotels (heating of interiors), hospitals, public building sites and retailing.
For many years people have been talking about “distributed generation” and “energy self-production”, i.e. energy production physically close to users.