What is combustion?

We explain what combustion is, how it occurs and what are the stages of the reaction. In addition, classification and examples.

1. What is combustion?

Combustion is a type of exothermic chemical reaction . It may involve matter in a gaseous state or in a heterogeneous state (liquid-gas or solid-gas). It generates light and heat and is produced more or less quickly.

Traditionally, combustion is understood as a process of rapid oxidation of certain combustible elements, that is, mainly constituted by hydrogen, carbon and sometimes sulfur. In addition, it necessarily takes place in the presence of oxygen .

In fact, combustions are forms of redox reaction (reduction-oxidation) that can occur both in a controlled manner, as in internal combustion engines, or uncontrolled, as in explosions. They involve the exchange of electrons between atoms of matter during the reaction. Therefore, they generate thermal and light energy.

In addition result other gaseous and solid substances such as carbon dioxide (CO ₂ ) and water vapor, or solid waste fuel (the substance consumed in the reaction) and the oxidizer (substance which promotes the reaction) , always according to its chemical nature.

Thus, although in the traditional image of combustion there is always fire involved, it is possible that this is not generated, since it is nothing more than a form of plasma (ionized gas) resulting from the release of heat from the chemical reaction.

2. How does combustion occur?

Combustions are a type of redox reaction , that is, oxidation reduction. This means that in them one material oxidizes (loses electrons), while the other one reduces (gains electrons).

In the case of combustion, the oxidizing agent (oxygen) obtains electrons from the reducing agent (fuel) , or what is the same, between the oxidizer and the fuel. This generally occurs according to the following formula:

Compound + O ₂ = Other compound + CO ₂ + H ₂ O + Energy

The compounds may vary, depending on their nature, as well as energy levels may vary . But carbon dioxide and water vapor are constant in all types of combustion.

3. Types of combustion

There are three types of combustion, which are the following:

• Complete or perfect combustion . Those reactions in which the combustible material is completely oxidized (consumed), and other oxygenated compounds are subproduced, such as carbon dioxide, sulfur dioxide or water vapor.
• Stoichiometric or neutral combustion . This is called the ideal complete combustion, which uses just amounts of oxygen for its reaction and that usually occurs only in the controlled environment of a laboratory.
• Incomplete combustion . Those in which compounds appear to oxidize (also called unburned) of flue gases, such as carbon monoxide (CO), hydrogen, carbon particles, and so on.

4. Combustion reaction

Combustion processes actually comprise a set of rapid and simultaneous chemical reactions, which may well be considered as one, which goes through the following phases or stages:

• Pre-reaction or first stage . Hydrocarbons present in the fuel material decompose and begin their reaction with oxygen in the air , forming radicals, which are compounds molecularly unstable. This initiates a chain reaction of appearance and disappearance of chemical compounds, tending to always create more than it destroys.
• Oxidation or second stage . In this stage most of the caloric energy of the reaction is generated, as oxygen reacts with the radicals of the previous stage, thus giving a process of violent electron displacement . A high number of radicals leads to a massive and violent reaction, known as an explosion.
• End of reaction or third stage . It occurs when the oxidation of the radicals is complete and the molecules of the released gases are formed . The combustible material will be completely depleted.

5. Combustion examples

Some simple examples of combustion in everyday life are:

• The lighting of a match / match . It is the most emblematic case of combustion. When the head of the match (phosphorus and sulfur cover) is scraped against a rough surface, it is heated by friction and triggers rapid combustion, which in turn produces a short flame.
• The lighting of a gas stove . Domestic kitchens work by combustion of a hydrocarbon gas, usually a mixture of propane (C ₃ H ₈ ) and butane (C ₄ H ₁₀ ), which the appliance extracts from a pipe or a container. Put in contact with the air, and provided an initial load of caloric energy (such as the pilot’s flame, or that of a phosphorus), the gas begins its reaction; but to keep the flame going, fuel must be supplied continuously.
• Strong bases and organic matter . Most strong bases (hydroxides) such as caustic soda, caustic potash and other extreme pH substances generate violent oxidation reactions when they come into contact with organic matter . This means that we can burn by contact and even start fires with them, since these types of reactions are usually very exothermic.
• The internal combustion engines . Present in cars, boats and other vehicles that operate with fossil fuel such as diesel, gasoline or kerosene, these devices are an example of controlled combustion. In them the hydrocarbons of the fuel are consumed and small explosions are generated that, within the piston system, are transformed into movement , also byproducing polluting gases, which are released into the atmosphere .