Science & Technology in Action

15th Edition

Natural gas and a low-carbon economy


When fuels are burned the usual products are carbon dioxide and water, and energy is released. Methane and natural gas produce the least amount of carbon dioxide for equivalent energy outputs. Another significant advantage of natural gas is that its combustion produces less particulates than liquid fuels, particularly diesel.

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Lesson excerpt

What is natural gas?

Natural gas is a naturally occurring hydrocarbon gas that is found in underground deposits, sometimes on its own but more often along with petroleum or coal deposits. Its composition varies considerably from place to place but its main constituent is methane (CH4). Methane is the lowest density hydrocarbon (55% of the density of air) and has the highest heat of combustion (55.6 kJ/g). It also has the lowest CO2 to energy output (56.9 g/MJ) of all fuels other than pure hydrogen.

Origin of natural gas

Natural gas, along with oil and coal, is formed by biological and thermal action on the organic remains of plants and animals, especially microorganisms. The two processes involved ― biogenic and thermogenic ― may take millions of years. The biogenic process takes place in shallow anaerobic conditions that are relatively cool (< 50°C). Bacteria convert the complex organic matter into a mixture of the lighter hydrocarbons, mainly methane, along with small amounts of ethane, propane and butane. The gas can seep out of the ground or sediment and may be seen to bubble to the surface in marshy areas. (The gases released in this process can sometimes ignite spontaneously producing a flickering light known as the “will o’ the wisp”.)

The thermogenic process takes place between 160°C and 220°C and under immense pressures ― tens or even hundreds of times atmospheric pressure. The hydrocarbons formed in these conditions range from gases to oils and tars.

The gases and oils formed in this way may be squeezed out of the rock layers and escape to the surface. Sometimes they are prevented from escaping by impervious layers (or ‘strata’) of rock, especially if the layers have been folded by geological processes to form a dome-shaped ‘trap’ or reservoir.

True or False?

  1. Methane is the main constituent of natural gas. true
  2. Natural gas is a naturally occurring carbohydrate. false
  3. Methane has a highest energy density than any fuel apart from hydrogen. true
  4. All fossil fuels are formed by biogenic processes. false
  5. Impervious layers of rock can prevent natural gas from seeping out of the rock strata. true
  6. Electricity is the one form of ‘primary energy’. false
  7. We use roughly equal amounts of energy for heating, transport and the generation of electricity. true
  8. When hydrocarbons are burned, carbon dioxide and water are produced. true
  9. For the same energy output, diesel fuel produces 50% more carbon dioxide than natural gas. true
  10. Hydrogen sulfide is added to natural gas to give it a distinctive smell. false

Glossary of terms

hydrocarbons in which there are only single bonds between carbon atoms
resulting from biological action
closed off; sealed
not porous; prevents leakage
microscopic solid particles of soot (mainly carbon); may be visible in high concentration as smoke
a thick blackish liquid or solid consisting mainly of hydrocarbons of high relative molecular mass
resulting from the action of heat
a space between rock strata from which oil or gas cannot escape