Conventional power stations burn coal, oil or gas to produce electricity. Coal, oil and gas are called fossil fuels because they form over millions of years through the decay, burial and compaction of rotting vegetation on land (coal), and marine organisms on the sea floor (oil and gas). Burning fossil fuels in this way releases large quantities of sulphur dioxide and nitrogen oxides which can cause acid rain.
Coal is a solid fuel formed over millions of years by the decay of land vegetation. Over time, successive layers become buried, compacted and heated, a process through which the deposits are turned into coal. Coal is widely used in the generation of electricity because it is a highly concentrated energy source. However, it is not a particularly "clean" fuel, releasing more acidic pollution than either oil or gas. Coal was the first fossil fuel to be exploited on a large scale during the 19th century with the beginning of the Industrial Revolution. Before the commercial introduction of electricity, coal was primarily used in industrial boilers to create steam energy to power machinery.
Oil is formed from the remains of marine microorganisms (microscopic animals and plants) deposited on the sea floor. As they accumulate over millions of years they gradually infiltrate the microscopic cavities of the sea floor sediment and rock where they decay. The resulting oil remains trapped in these spaces, forming oil reserves which can be extracted through large drilling platforms. The use of oil increased significantly after the Second World War. In the early 1970s, approximately 40% of global fossil fuel use came from oil, but during the 1990s this figure has decreased. Improved energy efficiency has caused oil consumption to decline in many developed, industrialised countries, as well as shifts to other fuels such as natural gas and nuclear energy. Decreasing use of oil is also resulting from tougher environmental restrictions concerning its use in some regions.
Natural gas is formed in the same way as oil, from the remains of marine microorganisms. From the mid-1960s, up until the present day, there has been a dramatic increase in the amount of proven reserves of natural gas. Consequently, natural gas has become the fastest-growing energy resource. The present global use of natural gas is approximately 20% of all fossil fuel use, and this figure is predicted to rise in the future. Natural gas provides an alternative to oil or coal in the provision of energy, and in terms of acidic pollution it is a cleaner fuel. Some estimates indicate the reserves of natural gas may be available for up to 400 years.