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Wood fire in a fireplace

A fuel is a chemical substance whose stored energy can be converted into usable energy through combustion .

Classification of fuels

Fuels are subdivided into solid, liquid and gaseous fuels according to various criteria, for example according to their physical state . Depending on their origin, a distinction is made between natural, refined or synthetic fuels. Fuels can be organic in nature, such as petroleum , natural gas, or coal , or inorganic in nature, such as hydrogen or carbon monoxide .

According to the type of energy released, fuels are divided into chemical fuels, which release thermal energy through oxidation , and electrochemical fuels, which are used, for example, in fuel cells to generate electricity .

The quality of a fuel is described by the calorific value or the calorific value . The calorific value describes the energy content of a fuel without the condensation heat of the water vapor formed during combustion, whereas the calorific value takes it into account, so the calorific value is lower by this amount of heat.

Fossil fuels are mostly formed over long, often millions of years, bio- and geochemical processes. The solid fuels were subject to coalification, the enrichment of carbon in organic matter enclosed by layers of the earth. The two main steps of coalification are the transition from organic matter, which mostly consisted of higher plant species such as wood or ferns, to brown coal and the further transition from brown coal to hard coal . Lower marine animals and plants are believed to be theorigin of the oil . The entire existing fuel reserves are called resources. These are divided into presumed and proven resources. Proven resources are further subdivided into degradable and presumably non-degradable resources. If the technical depletion of a resource is assured, this resource iscalled a reserve . The energy content of solid fossil fuels is often given in so-called hard coal units (SKE).

Today, a distinction is made between renewable raw materials , which are regularly reproducible resources ( biogenic fuels such as wood , biofuels , biogas ).

Nuclear fuels , such as enriched uranium and plutonium , which can release energy through nuclear fission or such as deuterium and tritium , which can release energy through nuclear fusion , are also not considered to be fossil fuels.

Physical state and use

Burning natural gas

Fuels are used in all three classic physical states ( solid , liquid and gaseous ). They are used for a wide variety of purposes.

  • Solid fuels , such as hard coal , lignite and wood , are primarily used nowadays to generate electricity in steam power plants . In addition, solid fuels are required for process engineering and the production of metal in the smelting of iron and steel. The importance for the heating of buildings is now only minor. The energy content is given in hard coal units (SKE).
  • Liquid fuels such as various petroleum derivatives ( gasoline , diesel , heating oil ) and biofuels play a significant role in transport and for heating buildings. They also serve as raw materials for products in the chemical industry . The energy content is given in oil units (OE).
  • Gaseous fuels such as natural gas and biogas are mainly used to heat buildings and generate electricity. They are also used as fuel and raw material in the chemical industry. Natural gas is also burned in small and medium-sized steam boiler systems to generate water vapor . The energy content is given in kWh / kg.

Fuel prices

The world market price for fuels fluctuates due to many factors. Measured in terms of the unit of hard coal , the price fluctuated in the period from 1970 to 2004:

  • for crude oil by 900 percent
  • for hard coal by 200 percent

The prices of the main fuels have a significant impact on national economies. Sharply rising prices can, for example, intensify existing inflation .

Fuel requirement

The determination of the fuel requirement is necessary in order to determine order or storage quantities, to estimate the consumption costs and to make comparative calculations with other energy sources or technologies. This applies to the overall economy as well as to an individual combustion system or consumer, be it stationary or mobile.

The fuel requirement depends, among other things, on: performance of the systems, efficiency, running time, fuel type, and special factors such as regulation and control.



Most of the fuels used in Germany have to be imported. This applies primarily to liquid and gaseous fuels, since Central Europe has only few resources of its own in this area. Solid natural fuels, on the other hand, are mined in larger quantities as mineral resources in this geographical area , with the costs incurred in the production being supported by subsidies in order to be able to keep the domestic fuel competitive with the prices paid on the world market .

The provision of fuels from renewable raw materials ( biogenic fuels ) is of growing importance for the energy industry and the transport sector. In 2009, 5.5 percent of the final energy required in Germany was provided by biogenic fuels (as bioenergy ). This means they make up around two thirds of renewable energies (10.1 percent in total). The majority of bioenergy is made up of heat. The total potential of bioenergy in Germany is limited, so that only part of the energy demand can be covered by it.


Web links

Wiktionary: fuel  - explanations of meanings, word origins, synonyms, translations
Commons : Fuels  - Collection of pictures, videos and audio files

Individual evidence

  1. Biofuels - basic data for Germany. ( Memento of the original from May 18, 2015 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. 14-page brochure, status: June 2010, Fachagentur Nachwachsende Rohstoffe e. V. (FNR), Gülzow 2009, online at Biomassehof-Achental.de, accessed on February 1, 2017 (PDF; 786 kB). @1@ 2Template: Webachiv / IABot / www.biomassehof-achental.de