Composition and technical properties
In terms of its composition, natural gas is a mixture of different gases or gaseous hydrocarbons, comprising mostly methane (CH4) (up to 99%), as well as heavy hydrocarbons such as ethane, propane, butane and ethylene, hydrogen sulphide, nitrogen and carbon dioxide, while its precise composition also depends on its source. The heavy hydrocarbons propane and butane are marketed as “liquefied petroleum gas” (LPG). LPG should be distinguished from liquefied natural gas (LNG).
LNG is natural gas which is cooled for purposes of transport to -161.6 degrees Celsius, where it changes from the gaseous to the liquid state. When it liquefies its volume decreases by a factor of 600, which makes transport over long distances economically feasible.
Other significant components of natural gas include hydrogen sulphide and carbon dioxide. Hydrogen sulphide is removed using a procedure called desulphurisation, while carbon dioxide is released into the atmosphere. Another common component of natural gas is nitrogen, which reduces the utility of the gas (lower calorific value, formation of nitrogen oxides).
Technical characteristics of natural gas:
- gross calorific value: (kJ/Sm3) → from 36,000 to 40,000
- Wobbe index (MJ/Sm3) → from 48 to 52
- density (kg/Sm3) → from 0.6 to 0.8
- specific gravity (air = 1) → from 0.54 to 0.59
- Sm3 = standard cubic meter (15 °C, 1.01325 bar)
- natural gas is a non-toxic, flammable, colourless gas that ignites at a temperature of around 600 °C; it is lighter than air.
From the source to the user
According to the International Energy Agency (IEA, 2011), conventional natural gas reserves that can be extracted using current technologies are 250 times larger than the current annual global consumption. According to the International Energy Agency’s World Energy Outlook in 2011, in view of current consumption trends there should be sufficient conventional and unconventional natural gas reserves to last for more than 250 years.
Natural gas is transported from the sources to the user in various ways: transmission via gas lines, transport of liquefied natural gas by LNG tankers and transport of pressurised natural gas by train. Transmission via gas lines (which include low, medium, high and ultra high-pressure lines) is the most energy efficient, and is economically justified when there are large quantities of gas being transported.
The transmission of natural gas is also closely linked to storage. Underground storage facilities enable the storage of natural gas for subsequent periods of higher consumption or as a reserve for unforeseen events, such as long delays in supply.
Various components have to be removed from natural gas before it can be released into the network, primarily heavy hydrocarbons, water and in some cases also sulphur. The flow of natural gas via pipelines is made possible by compressors, which increase the pressure in pipes and push the gas forward. Compressor stations are located every 70 to 180 km. Natural gas is odourless, and therefore small quantities of odorant are added before it is fed into the distribution network (to which households are connected), in order to make it easier for us to detect the presence of natural gas indoors.