CRUDE OIL is formed from organic material of the remains of plant and animal organisms that lived millions of years ago.
These remains form sediments e.g. at the bottom of seas, and are buried under layers of sedimentary rock. They are transformed, without O2, under the action of heat and pressure to form crude oil over millions of years.
The vast majority of compounds found in crude oil are hydrocarbons, – compounds/molecules made up of carbon & hydrogen atoms only.
Oil is formed from once living organisms and the Sun is therefore the original source of energy. It is a non-renewable and finite (limited reserves) energy resource because it takes millions of years to form and we burn it faster than it is formed.
It is a finite energy resource because it will eventually run out! We do not have unlimited oil reserves!
Coal, peat and natural gas are the other principal non-renewable fossil fuels formed from the remains of plants or animals.
Coal forms by accumulation of dead land-based plant life, mainly trees. This organic matter was deposited in sedimentary basins on land (of continental origin), where the water was shallow. These basins were either close to the sea, often in the form of large lagoons, or inland, in the form of lakes or marshes. As a result of climatic variations, for example an increasingly heavy annual rainfall, it is thought that enormous forests sank below water, and their debris accumulated in sedimentary basins where it was rapidly covered by large quantities of mud and sand.
This sudden and premature burial sheltered the debris from the air, thus preventing it from rotting quickly. In the maritime areas, a sudden subsiding of the basin resulted in an inflow of seawater, decimating the forest. After these catastrophic episodes, the forest grew again until a new disaster happened, and so on. This repetitive cycle of events explains why, in the substratum, layers of coal alternate with layers of clay or compacted sandstone and).
Coal mainly consists of carbon. Burning coal produces a lot of pollution as the greenhouse gas carbon dioxide. The pollutants include soot particles (black deposits of carbon), sulphur dioxide (lung irritant and acid rain gas) and poly-aromatic hydrocarbons which are carcinogenic.
Natural gas, mainly the hydrocarbon methane CH4, is often found with oil. It consists of 25% hydrogen and 75% carbon, and, apart from the ‘greenhouse’ CO2, produces far less pollution than coal on combustion.
Peat (‘turf’) is formed much more quickly, over a few thousands of years, from the decay of plant material in the absence of oxygen, in boggy waterlogged ground. It is a poor quality fuel since the carbon content is much less than in coal and large amount of ash formed on combustion. However, there is a peat fired power station in Ireland, where peat is plentiful, hence fuel transport costs are kept to a minimum.
THE CARBON CYCLE.
When the fossil fuels are burned the ‘carbon’, as carbon dioxide, is returned to the atmosphere of the Earth’s environment. There, it gets absorbed by plant leaves and used up in photosynthesis with the help of sunlight energy and green chlorophyll. The plant material decays reforming carbon dioxide, or, is eaten by animals and used in respiration to form carbon dioxide. Either way, this completes the carbon cycle.
Photosynthesis: carbon dioxide + water -> glucose + oxygen
6CO2 + 6H2O -> C6H12O6 + 6O2
Respiration: glucose + oxygen -> carbon dioxide + water
C6H12O6 + 6O2 -> 6CO2 + 6H2O
Greenhouse gases in the Earth’s atmosphere, such as carbon dioxide or methane, absorb the long IR radiation reflected off the surface of the Earth, having passed though the atmosphere from the sun. The CH bond resonates at the frequency of this IR, which means it absorbs the energy efficiently and re-radiates longer wavelength IR in all directions, some of which being back to Earth, which warms the surface. We are adding greenhouse gases artificially from industrial pollution – the so-called ‘enhanced greenhouse effect’ – thus contributing more to planetary surface warming.