Fuel

Fuel is any material that can react with other substances to release energy. This energy is often released as thermal energy, which is heat. It can also be used for work or to provide light through combustion. While we often think of fuel as something we burn, the concept includes other energy sources. For example, nuclear energy is used through processes called nuclear fission and nuclear fusion. The heat from these reactions can be converted into mechanical energy using a heat engine. This allows us to turn heat into movement for machines.

Chemical fuels release energy through a specific process called combustion. This is a reaction where the fuel interacts with substances around it. We can classify these chemical fuels in two different ways. First, we look at their physical state: solid, liquid, or gas. Second, we look at how they occur in nature. Primary fuels are natural, while secondary fuels are artificial. For instance, wood is a primary solid fuel, but charcoal is a secondary solid fuel made from wood.

Solid fuels are materials used to produce heat and energy, usually by burning. Common examples include wood, coal, peat, and dung. Some people also use pellets made from grains like corn or wheat. Coal was particularly important because it powered the Industrial Revolution. It allowed people to fire large furnaces and run steam engines. Today, coal and peat are still used to generate electricity. However, some cities limit the use of solid fuels like coal because they release toxic emissions.

Liquid fuels are molecules that can be used to create mechanical energy. Most of these fuels are derived from fossilized remains of ancient plants and animals. These remains were transformed by heat and pressure inside the Earth's crust. Liquid fuels like gasoline, diesel, and kerosene are very important for transportation. They are easy to transport and handle in many engineering applications. Some liquid fuels, like hydrogen or ethanol, are different from traditional petroleum products.

Fuel gases are fuels that exist as gases under normal conditions. Many of these are hydrocarbons, such as methane or propane. One major advantage of gas is that it can be moved through pipes directly to where it is needed. This makes it easier to distribute than solid or liquid fuels. However, gas can be dangerous because it can collect in areas undetected. To prevent accidents, workers add odorizers so people can smell a leak. Natural gas is the most common type of fuel gas used today.

Fossil fuels are a specific group of hydrocarbons like coal, petroleum, and natural gas. They formed over hundreds of millions of years from dead plants and animals. This process happened deep underground without oxygen. In 2007, fossil fuels made up 86.4% of the world's primary energy consumption. Petroleum accounted for 36.0%, coal for 27.4%, and natural gas for 23.0%. Because they take millions of years to form, they are non-renewable resources. This means we are using them much faster than nature can replace them.

Using fossil fuels has significant effects on our planet. When we burn them, they release about 21.3 billion tonnes of carbon dioxide each year. Natural processes can only absorb about half of that amount. This creates a net increase of 10.65 billion tonnes of carbon dioxide in the atmosphere. This process is linked to global warming. Because of these environmental concerns, there is a global move toward renewable energy. This includes biofuels, which come from biomass like plants that can be replenished quickly.