Fuels are materials that create usable energy through chemical, nuclear or electrochemical reactions. The evolution of energy is usually controlled so that it may be used for heating or converted into some other form of energy, such as electricity. Most commonly used fuels are derived from hydrocarbon materials and release their energy in the form of heat during the process of Combustion. The most important fuels, not derived from hydrocarbons, include the nuclear fuels, uranium and plutonium, and liquid hydrogen for use in fuel cells and rocket engines. Fuels may be solid, liquid or gaseous and can be classified as primary or secondary. A primary fuel occurs naturally in a form that is suitable for immediate use, such as coal, whereas a secondary fuel is derived from a naturally occurring material by some form of processing that has radically altered it, such as coal gas from coal. Carbonaceous fuels may be further classified as fossil or non-fossil. Fossil fuels are derived from the decay of organic matter over millions of years and are the most extensively used group of fuels around the world.
The three major fossil fuels are coal, petroleum and natural gas. Coal has been used extensively throughout the industrialized world since the beginning of the industrial revolution. It can be mined from deep under the ground or deposits close to the surface can be strip-mined. The latter technique is used commonly around the world and provides the most economic source of coal. Coal requires very little processing before use. Some washing may be required to remove rock followed by crushing as necessary for the particular use. The major use for coal is combustion to provide electricity or heat, but it is also used to make coke for the production of iron and steel and for some production of chemicals. Coal occurs in various forms depending on its geological source and degree of coalification or rank. Of the three major fossil fuels coal has the highest carbon-to-hydrogen ratio and the lowest calorific value. The calorific value (on a mineral matter free basis) varies with rank from about 23 MJ/kg for the lower rank coals to about 34 MJ/kg for the higher rank and bituminous coals. Despite its solid form, coal is traded extensively around the world and still finds the greatest use of all fossil fuels for the generation of electricity. Its great advantage is that there are known deposits available for many hundreds of years and the deposits are found all around the world in all continents, hence giving a competitive world market and stable prices.
The use of petroleum has increased significantly since the 1950s with massive increases in the use of transport fuels, which are almost exclusively supplied from petroleum, and the growth of the petrochemical industry. Petroleum, or crude oil, is extracted, as a liquid, from wells on both land and sea. (See Oils.) As with coal, there are different forms of crude from different parts of the world. However, in all cases the erodes are processed (i.e., refined) to give a variety of liquid products which find different applications. As mentioned above, many of these are for transport fuel, such as petrol, diesel and aviation spirit. But some fractions, such as gas oil and fuel oil, are used for combustion for heating and the generation of electricity. Fuels, such as gas oil, are very convenient for industrial heating purposes since they are clean liquids which can be easily transported, stored and pumped. However, the cost of such fuels compared to others may make them less attractive and the economic usage of a particular fuel will depend very much on individual circumstances. Petroleum-derived fuels are attractive for use since they are in the liquid form and they have a lower carbon-to-hydrogen than coal and higher calorific values ranging from about 45-48 MJ/kg. There are sources of petroleum around the world and new finds are being made. However, there is still a concentration of resources in the Middle East, which could, in certain circumstances, influence the petroleum market and prices.
Natural Gas resources have a recent history of usage. Natural gas is mainly Methane and has the lowest carbon-to-hydrogen ratio of the fossil fuels and the highest calorific value of about 55 MJ/kg, although this value will be lower if the methane content in the gas is lower. Natural gas is a highly convenient and "clean" form of fossil fuel requiring minimal treatment before use, and is used for combustion for both heat and electricity generation. It is very applicable to local use where a network of distribution pipelines exists. However, it is a difficult fuel to transport over distances as a world commodity in the same way as coal and petroleum and requires expensive pipelines or very specialized shipping facilities.
The major fossil fuels mentioned above represent a very large part of the fuel usage throughout the world. However, other energy sources are used, such as nuclear and hydro, for the generation of electricity and these can be major sources in some local areas and countries. (See Hydropower and Nuclear Reactors.) There are also emerging sources of fuels which include renewables such as biomass, where the main sources are: wood; municipal solid wastes; agriculture and industrial wastes; methane from landfill gas; and ethanol. (See Alternative Energy Sources.) There are also existing or developing technologies which convert one particular fuel into another. This could be coal into liquids or gases, biomass into liquids or gases; and gases into liquids. Conversion has the advantage of meeting local needs, converting a fuel into a more transportable form (such as gas to liquid) or as a solution to environmental problems. For example, the gasification of coal yields better methods for the removal of pollutants such as sulfur and nitrogen, and enables the coal to be used more efficiently in combined cycles.
For the engineer or scientist there is a great deal of literature on the composition, properties and usage of different fuels. Some references are given below which may prove useful in finding information.
Technical Data on Fuel, 7th edn., J. W. Rose and J. R. Cooper Eds., (1977) Scottish Academic Press.
Fuels and Fuel Technology, 2nd edn., W. Francis and M. C. Peters, Pergamon (1980).
Fuel Science and Technology Handbook, J. G. Speight Ed., Chemical Industries Series, No. 41, (1990) Dekker.
Encyclopedia of Chemical Technology, 4th edn., 11 and 12, J. I. Kroschwitz Ed., (1993) Wiley-Interscience.