In a process fuel reacts with other substances and creates chemical or nuclear energy. This process is often a combustion reaction, but for example in a hydrogen fuel cell no combustion occurs. The only reaction happening in such a device is the oxidation of H2. Energy created from these exothermic reactions can be used as heat or work. Fuel can be distinguished into two different groups, namely chemical and nuclear fuel.Chemical fuel is often used in processes such as combustion, where two substances react with each other and release energy. The majority of the energy in fuel is stored in the weak double-bond of oxygen and not in the chemical bonds of the fuel. Chemical energy can be further divided on basis of their physical properties, as solid, liquid or gas and on basis of their occurrence, meaning whether the fuel is natural or has been created artificially.Solid fuels such as coal and wood, provide energy by combustion and is often released as heat. For many centuries solid fuels have been used majorly. During the industrial revolution coal was used to run the machines. Nowadays solid fuels have been replaced by liquids or gases. Solid fuel types are also prohibited in a lot of urban areas, because of the emissions and pollution that are produced by solid fuels. For example, burning coal does produce a lot of energy, but creates more soot and other pollutants compared to burning liquid or gaseous fuels.For the last century liquid and gas fuels are being used more compared to solid fuels. The major advantages of liquid and gaseous fuels is that they are more easy to transport and use, the downside is that they are more flammable and thus less safe. Most liquid fuels are obtained from fossilized remains from dead plants and animals. Whereas liquid fuels are often a product from fossils, gaseous fuels are derived from artificial processes, which often use raw oil as a resource. Compared to solid and liquid fuels, gaseous have some advantages, they are for example very easy to use because no pump is needed to move the fuel. The gas will be moved by the high pressure they are usually transported at. However gases also have downsides. They need to be transported at high pressures in order to increase the energy content per litre. This can be dangerous and lead to unwanted explosions. Another downside is that gases can easily go undetected and collect in certain areas. This can for instance lead to a gas explosion or suffocation of living organisms and most importantly, people. Scientists have come up with a solution, namely adding an odour to the gas, so people can detect a gas by its smell and thus react when a gas leak occurs.. Besides different physical properties, fuels also release different amounts of energy. In the table below the most commonly used fuels have been shown.