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A fuel is a chemical substance whose stored energy can be converted into usable energy through combustion.
Fuels are categorized according to various criteria, such as their physical state into solid, liquid and gaseous fuels. Depending on their origin, a distinction is made between renewable, natural, refined or synthetic fuels. Fuels can be organic, such as crude oil, natural gas or coal, or inorganic, such as hydrogen or carbon monoxide. Depending on the type of energy release, fuels are divided into chemical fuels, which release thermal energy through oxidation, and electrochemical fuels, which are used, for example, in fuel cells for coupledpower generation. Fuels can also be fossil fuels or fuels from renewable raw materials (biogenic fuels such as wood, biofuels, biogas). Nuclear fuels, such as enriched uranium and plutonium, which can release energy through nuclear fission, or deuterium and tritium, which can release energy through nuclear fusion, are not fossil fuels.
The quality of a fuel is characterized by the heating value or the calorific value.Fossil fuels are usually formed over long bio- and geochemical processes, often lasting millions of years. The solid fuels underwent carbonization, the accumulation of carbon in organic matter sealed by layers of earth. The two main stages of coalification are the transition from organic matter, which mostly consisted of higher plant species such as wood or ferns, to lignite and the further transition from lignite to hard coal. Lower marine animals and plants are thought to be the origin of the crude oil.
The total existing fuel reserves are called resources. These are divided into presumed and proven resources. Proven resources are further subdivided into mineable and presumably non-minable resources. If the technical extraction of a resource is assured, this resource is called a reserve.The energy content of solid fossil fuels is often specified in so-called coal equivalent units (HCU)The information on ranges different fossil fuels varies greatly depending on assumptions about the development of demand and supply as well as undiscovered deposits. If supply becomes scarce, the price will rise: less favorable deposits (e.g. oil sands deposits) can then be additionally developed, as the price increase brings them into the area of profitability. For example, Canada moved up to 2nd place among the countries with the richest oil reserves following the revaluation of its oil sands deposits after the rise in oil prices as a result of the Iraq war. [vistaverde, 31.1.03] Forecasts about the range of fossil fuels are therefore very uncertain and are constantly being revised. For petroleum, for example, with estimated reserves of 165 billion tons and extrapolation of annual consumption in 2002 of 3.3 billion tons, the range is 50 years. (Further data: => Globus infographic 0395 ). The Bundesanstalt für Geowissenschaften und Rohstoffe (BGR) estimates the "static range", i.e. the range of crude oil with constant consumption, at around 43 years.
Sources:
Agenda21-meeting point http://www.agenda21-treffpunkt.de