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Aerial view of a car on a road in the middle of a forest

What types of fuels are there?

Energy for our everyday life

Nowadays, life without fuels would be unimaginable: They provide us with electricity and heat, allow us to move from one place to another comfortably, power the transport of goods, and feed the manufacturing processes of a huge variety of essential products in our society, from steel to sanitary products or plastics. Given their enormous importance, we will now see what types of fuels exist and how they are classified.

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Fuels based on their physical state

Depending on their physical state, fuels can be classified as solid, liquid, and gaseous.

  1. Solid: Solid fuels have a compact appearance. Some examples include coal, wood, or biomass (which includes residues from the processing industries for wood, pulp, and paper, as well as agricultural residues). Of these energy sources, coal has great historical importance, since it has traditionally been used both to generate energy and to heat enclosures and rooms. However, its combustion has certain disadvantages, such as the emission of polluting and greenhouse gases.

  2. Liquid: Liquid fuels are characterized by being in this state at room temperature and standard atmospheric pressure. Petroleum is the best known, but it is not usually used in its crude oil state, but is refined to produce gasoline, diesel, or kerosene, among others. They are mainly used in transportation, and their combustion also emits polluting and greenhouse gases. However, there are solutions that help to reduce their environmental impact and maximize their efficiency.

  3. Gaseous: These fuels exist in the gas state; the most prominent examples are natural gas —which is mainly made up of methane— and liquefied petroleum gases (LPG), which come from mixtures of hydrocarbons such as propane and butane, among others. They are often used to generate electricity, for heating, and for fuel for vehicles. Furthermore, they pollute less than solid or liquid forms of fuel.

Fuels based on their origin

An illustration depicting various types of renewable fuel
  • Fossil fuels

Formed from the decomposition of organic matter over millions of years, the main fuels of this type are petroleum, coal, and natural gas. These are finite and non-renewable fuels, so the amount available is limited and, furthermore, it is not distributed evenly across the planet.

These fuels are produced from raw materials of renewable origin. For example, the organic waste generated by our own society, such as used cooking oils, agricultural waste, livestock waste, or waste from the agri-food industry. They can also be made by combining CO₂ captured from the atmosphere and renewable hydrogen. One of their advantages is that they have net zero emissions because the amount released when using them is equivalent to the CO₂ that has previously been removed from the atmosphere for their manufacture. For this reason, they are one of the most effective alternatives to reduce transport emissions in the short term.

Fuels depending on their use

Fuel pump at a gas station

For transport

The most used are gasoline and diesel and are obtained by distilling crude oil. The former is used in vehicles such as cars, motorcycles, and lightweight trucks, while diesel is used in cars and mainly in heavyweight trucks and buses. For its part, kerosene has gone from being a fuel for street lamps to jet fuel and is the cheapest source of energy.

Another fuel that is increasingly used is Autogas or LPG (Liquefied Petroleum Gas), a fuel whose result is the mixture of butane and propane gas and that millions of vehicles use due to their environmental benefits.

Renewable fuels are also expanding, Today, the gasolines and gasoils that we refuel in Spain already contain 10% renewable fuel. The most common are biodiesel (mixture with conventional diesel) and bioethanol (mixture with gasoline) and can be used in car, truck, airplane, and boat engines with hardly any modifications.

Wood pellets, birch, and ramp with leaves

For heating

Gas —whether it be butane, propane, or natural gas— and gasoil have a higher calorific value than electricity, making them more energy efficient. In addition, they are very convenient to use, provide fast results, and are very safe.

On the other hand, biomass has recently become popular (wood chips, pellets, or agricultural waste) for heating. It is either burned directly in boilers or used in central heating systems to generate both heat and domestic hot water.

Solar panels and wind turbines on the sea

For electricity generation

Fossil fuels such as natural gas, coal, and fueloil have been used for years to generate electricity in conventional thermal power plants. Their combustion produces steam and drives the turbines that generate electricity.

Natural gas is also used in combined cycle thermal power plants, where its thermal energy is transformed into electricity with the help of a gas and steam turbine. These power plants are more efficient, since they consume less fuel, waste less water, and have lower emissions.

Synthetic fuels

What are they? How are they produced? What advantages do they have? Discover all the answers to these questions from Repsol.

Go to synthetic fuels