Such natural fuels as coal, petroleum, and natural gas are obtained from underground deposits that were formed millions of years ago from the remains of plants and animals. These fuels, which are called fossil fuels, account for about 90 per cent of the energy people use today. Synthetic fuels can be made from fossil fuels, certain types of rock and sand, and biomass. Biomass is the name given to such replaceable organic matter as wood, garbage, and animal manure that can be used to produce energy. Some synthetic fuels are made from chemicals.
Most fuels release energy by burning with oxygen in the air.
Oxidizers are compounds that contain oxygen. Since the 1970's, shortages of some fuels and concerns about the environmental effects resulting from the burning of fuels have led people to explore other sources of energy. For information on the availability of fuels, their effect on the environment, and alternative energy sources, see Energy supply and Environmental pollution . Solid fuels Coal is used chiefly to produce electricity. It is burned to create heat to turn water into steam.