What’s combustion?

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Combustion requires fuel, oxygen, and heat to start a chemical reaction. Fuel contains potential energy, and each fuel has a specific energy density. Oxygen can come from air or other oxidizing agents. Heat is what starts the combustion process, and once it starts, additional heat is not always needed. Combustion can release exhaust, including particulates that can contribute to air pollution. The combustion process can be controlled by increasing or decreasing any of the three parts. The best way to put out a fire depends on the type and size of the fire.

Simply put, combustion means to burn. For the combustion process to take place, fuel, oxygen, and an ignition heat source are needed to start a chemical chain reaction; in a campfire, for example, the wood is the fuel, the surrounding air supplies the oxygen, and a match or lighter can start the fire. Increasing one of these elements will increase the intensity of the fire, while eliminating one of them will cause the process to stop. If the fire is smothered with water or dirt, for example, the oxygen can no longer reach the heat and fuel and it goes out.


Fuel is the substance that burns during the combustion process. All fuels contain chemical potential energy; this is the amount of energy that will be released during a chemical reaction. How much energy a substance releases as it burns is called the heat of combustion. Each fuel has a specific energy density, i.e. how many megajoules (MJ) of energy are produced per kilogram (kg) of substance; methane, for example, has an energy density of 55.5 MJ/kg, meaning it can deliver more energy than sulfur at 9.16 MJ/kg.

A wide variety of substances can be used as fuel, but hydrocarbons are among the most common. These include methane, propane, gasoline and jet fuel, to name a few; all fossil fuels, including coal and natural gas, are hydrocarbons. Other substances commonly used as fuels include hydrogen, alcohol and biofuels, such as wood.

During combustion, fuel is transformed into heat and exhaust. For example, when gasoline burns, it produces water (vapour), carbon dioxide, nitrogen, carbon monoxide and other elements. Combustion can also release particulates, which are tiny particles that float in the air; those released by burning fossil fuels and wood often contribute to air pollution. However, the exhaust can be used for beneficial purposes, such as providing the thrust that propels a rocket through the air. Most exhaust gases are in the form of gases due to the heat produced by the combustion process, but they can also be in liquid or solid form.


For fuel to burn in the combustion process, it must also contain oxygen. The most common source is air, which contains about 21% oxygen. Other sources, often known as oxidants or oxidizing agents, include hydrogen peroxide, potassium nitrate, and many others. When an oxidizing agent is introduced into a fuel, it releases oxygen and can increase the rate at which a fire burns.

Like fuel, oxygen doesn’t have to be in the form of a gas, although it is very common. In a solid rocket, for example, a solid oxidizer is mixed with the fuel to create propellant, which burns when ignited and propels the rocket forward. The space shuttle and other spacecraft use liquid oxygen as part of the combustion process.
When a fire doesn’t have enough oxygen, it doesn’t burn completely. This incomplete combustion produces carbon monoxide, carbon (soot) and other particulates that contaminate the air. Incomplete combustion in a fireplace or home furnace can release toxic gases and be very dangerous.


Heat or ignition is what starts the combustion process. Since heat is also produced when something burns, once the process has started, additional heat is not always needed to keep the chemical chain reaction going. The initial spark that starts the chemical process can be provided by a flame, friction or even the heat of the sun.
In the event of spontaneous combustion, fermentation or oxidation can create enough heat to start a fire. In a compost pile, for example, bacteria can start breaking down organic compounds, creating enough heat and oxygen to cause combustion. Some materials, called pyrophoric substances, ignite when exposed to air or water; phosphorus and plutonium are two examples. When these materials encounter a fuel source, they can start a fire that is very difficult to put out.

Combustion process control
Since all three parts are required for combustion, increasing or decreasing any of them will affect the process. Increasing the amount of oxygen added to a fire using an oxidizing agent, for example, will make the fire burn faster. Removing or reducing the fuel source will cause it to burn smaller or go extinct.
There are three basic ways to stop the combustion process:

take away the fuel,
remove oxygen,
and/or turn off the heat.
Combustion can also be stopped by stopping the chemical chain reaction that creates the flames. This is especially important when some metals, such as magnesium, burn because adding water to the fire will only make it stronger. In such cases, dry chemicals or halomethanes are used to stop the reaction.
Which of these is the best way to put out a fire depends on the type and size of the fire. In a house fire, for example, firefighters use water or foam to keep oxygen from reaching the fuel and to reduce the temperature. While water may be used in a forest or bushfire, removing new fuel for the fire by clearing brush and dead vegetation from the area is often a big part of stopping it.

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