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Plants store the energy of the sun in sugars made through the process of photosynthesis. Carbon dioxide from the air and water from the soil are combined in the presence of chlorophyll and sunlight into sugar.

The energy that is stored in the chemical bonds of the sugar molecule is then used in a process called respiration to supply the needed energy for other cellular processes.

Animals and other living organisms that are incapable of photosynthesis obtain stored energy from plants as food.

Normally when plants or animals die, they decay totally. The energy stored in the chemical bonds and the elements in the compounds are returned to the environment. If, however, the plant or animal is covered shortly after it dies under anaerobic conditions, it can only partially decompose because of the lack of oxygen. That which remains contains some of the stored potential energy in its chemical bonds. Peat is a good example of this phenomenon. Peat moss grows in wet bogs and when the plants die they are covered with water and other dead plants. The accumulated mass of partially decomposed peat can be used as a fuel.

During the Carboniferous period, ca. 300 million years ago, the climate was much more tropical over vast portions of the Earth. Large swamps and jungles existed, ideal for plant growth. When plants died, they were often covered by mud and other plants where only partial decomposition was possible. The weight of accumulated material pressed the plant remains and geologic processes slowly converted them into coal. This coal is mostly carbon and contains energy that was stored by the living plants from which it was made. It is this energy that is liberated when the coal is burned, returning the carbon to the environment as carbon dioxide.

Closely related to coal is petroleum which is the remains of marine micro-animals and plants that were only partially decomposo-animals and plants that were only partially decomposed when they died. Geologic processes have turned their remains into liquid consisting mostly of hydrocarbons (compounds made up of carbon and hydrogen atoms). Petroleum (also called crude oil) is trapped under some rock formations and is obtained by drilling wells and then pumping the liquid fuel from out of the ground. Found with petroleum in many locations is a gas called natural gas. Natural gas is composed mostly of methane, CH4, a clean burning fuel.

Coal, petroleum, and natural gas are the fossil fuels. The energy they contain is used when these fuels are burned to supply heat for a variety of uses. Over 90% of all the energy that man uses comes from these fossil fuels.

Coal is often burned directly to produce heat. Or, coal can be processed to produce other useful fuels and products. When coal is heated in the absence of air, three useful materials are produced, a gas, a liquid, and a solid. The gas that is liberated from the heating of coal in the absence of air is called town gas or coal gas. It is a mixture of hydrogen, methane, and carbon monoxide. Town gas has the advantage over coal that it can be delivered to home and factories through pipes and is a cleaner burning fuel than coal itself. Town gas is used to heat water, cook food, provide space heat, and at one time was used for lighting. Town gas is popular in places where there iplaces where there is coal but no natural gas source. The liquid that distills from heated coal is called coal tar. It is a complex mixture of organic compounds. At one time coal tar was considered to be a nuisance and was one of the first toxic wastes. Today coal tar is an important source of organic compounds that are used in the synthesis of dyes, plastics, and many other important manmade materials. After town gas and coal tar have been distilled off the coal, a solid remains. This material is almost pure carbon and is called coke. It is used in the production of steel, or as a clean burning fuel.

Petroleum is not used directly as a fuel, it is first refined into a series of products. Subjected to distillation, it is separated into liquid fractions of hydrocarbons with different boiling points and properties. The first use of petroleum was in the production of kerosene as a substitute for whale oil used in lamps. Below is a table summarizing the products produced from petroleum by distillation or refining:

Fraction Distillation

temp, oC

gas below 20
petroleum ether 20-60
ligroin (Light naphtha) 60-100
gasoline 40-205
kerosene 175-325
gas oil above 275
lubricating oil non-volatile liquids
asphalt non-volatile solids

These various fractions are used as fuels, solvents, and sources of organic compounds from which plastics, medicines, paints, dyes, and other useful materials are made.

Also see: Octane Rating ating