The term fuel refers to any substance which is burned to produce energy, typically for heating, cooking, or producing mechanical work in a motor.
- Coal was the most important fuel during the 19th century, and continues to be widely used in China and in the steelmaking industry.
- From Petroleum (which is a naturally occuring liquid), several fuels (called petrofuels) are derived including LPG, butane, gasoline, jet fuel, kerosene, fuel oil and Diesel fuel. This liquid has been sequestered since hundreds of millions of years, meaning that when burned they reintroduce CO² into the atmosphere (hereby contributing to global warming). As petrofuels have been so widely used in the 20th and 21st century, all other fuels are called "alternative fuels".
- Hydrogen, oxyhydrogen, nitrous oxide, syngas, compressed air and other zero-emission (ZE) gases are currently on the rise. Most of these (hydrogen, oxyhydrogen, syngas ) have high amounts of energy density, but storage of the gases is still very problematic. Nitrous oxide has high amounts of energy density, and is easy to store yet can't be burned that easily. Compressed air is the only gas that has but little energy density per m³ of gas.