4.6 - What is a Fuel

A fuel is any substance that can be burned to release energy in the form of heat and light. Fuels are used in various industries, transport systems, and in homes to produce energy for different purposes. Fuels are typically categorized based on their physical state, composition, and whether they are renewable or non-renewable.

Fuels are substances that undergo combustion—a chemical reaction with oxygen in the air to produce energy. Combustion is an exothermic reaction, meaning it releases heat. This energy is useful for heating, cooking, generating electricity, and powering vehicles.

Fuels can be divided into two main categories:

• Natural Fuels: These include fossil fuels such as coal, oil, and natural gas. They are derived from natural sources and have been formed over millions of years from the remains of plants and animals.
• Man-made Fuels: These are fuels that are artificially manufactured or processed. Examples include alcohol (like ethanol), wood pellets, and biogas.

Fuels are characterized by several properties that determine their suitability for various applications:

• Calorific Value: It refers to the amount of heat energy produced when a specific amount of fuel is burned. Calorific value is typically expressed in Joules per gram (J/g) or kilocalories per gram (kcal/g). Higher calorific value fuels are more efficient in energy production.
• Ignition Temperature: The minimum temperature at which a fuel will catch fire without the need for an external flame. Different fuels have different ignition temperatures, with solid fuels like coal requiring higher temperatures compared to liquid fuels like petrol.
• Flammability: This refers to how easily a fuel catches fire. Fuels like hydrogen and natural gas are highly flammable, while substances like water or sand are non-flammable.

The combustion of fuels involves a chemical reaction between the fuel and oxygen. For example, when methane gas ($CH_4$) burns in oxygen, the reaction is as follows:

$CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O + \text{energy}$

In this reaction, methane ($CH_4$), a fuel, reacts with oxygen ($O_2$) from the air to produce carbon dioxide ($CO_2$) and water ($H_2O$) along with the release of energy in the form of heat and light.

Fuels can also be classified based on their renewability:

• Renewable Fuels: These are fuels that can be replenished naturally in a short period. Examples include biofuels like wood, biogas, and ethanol. Since these sources can regenerate relatively quickly, they are considered sustainable.
• Non-renewable Fuels: These fuels, such as coal, petroleum, and natural gas, are finite and take millions of years to form. Once depleted, they cannot be replenished within a human lifespan, making them unsustainable in the long run.

During combustion, if oxygen is insufficient, incomplete combustion occurs. This results in the formation of carbon monoxide ($CO$) and soot instead of carbon dioxide. Incomplete combustion is dangerous as carbon monoxide is a poisonous gas.

Fuels are essential for various human activities, but their use has environmental consequences, especially when non-renewable fuels are burned. The release of pollutants such as carbon dioxide contributes to global warming and climate change, leading to the development of alternative fuels that are more environmentally friendly.