Combustion is generally understood as synonymous with burning, but from a chemical point of view, combustion is a much broader phenomenon. Combustion can be defined as a highly exothermic oxidation reaction, which means that it is a reaction that releases energy in the form of heatsometimes accompanied by light, as in fire.
Although combustion is an exothermic reaction, its initiation may require the input of heat until it reaches temperature. ignition temperature, a characteristic of each substance. Once combustion begins, the very heat released by the reaction is used to continue it.
Fuels and oxidants
As in any reaction, there are some substances that react with each other, the substrates, to form other substances, the products. Combustion substrates are called fuel and oxidant:
- gas: it is the oxidizable substance, the one that «burns».
- oxidising: is the oxidizing substance, which causes or favors the combustion of fuel (not to be confused with fuel, which is a type of fuel).
Fuels are substances that give off heat when oxidized and some of the most common are hydrocarbons, both gaseous and liquid and solid. For example, butane is a gaseous hydrocarbon, diesel fuel is a liquid, and coal is a solid. Among the oxidizers, the most common is the oxygen in the air.
The products derived from combustion depend on the fuels and oxidants involved. In the most typical combustion, that of a hydrocarbon and oxygen, the carbon-hydrogen bonds in the hydrocarbon are broken and both elements combine with oxygen to form carbon dioxide (COtwo) and water vapor (HtwoEITHER).
For example, in the combustion of methane (CH4) with oxygen the following reaction occurs:
Since CH bonds contain more energy than CO and HO bonds, there are energy that is released and is the cause of the heat.
Combustion is a essential reaction for many forms of life of our planet, including human beings, who use the controlled combustion of nutrients at the cellular level as a source of energy. For example, the mitochondria use the energy released by the combustion of glucose to form ATP, a molecule that the cell can use later in its vital processes that require energy input.
Complete combustion and incomplete combustion
Looking at the previous example of the combustion of methane, you can see that it takes two molecules of Otwo for the combustion of a molecule of CH4. If there is enough oxygen, there will be a complete combustion in which all the methane is transformed into COtwo and HtwoEITHER.
The relationship between the amount of fuel and oxidant for complete combustion is the stoichiometric ratio. In the example of methane and oxygen, this ratio would be 1:2. Below this level, the combustion of a material can continue to occur but there will come a time when the oxygen is depleted and the reaction stops or is partially. Combustion will have been incomplete.
In the incomplete combustion of hydrocarbons, in addition to producing COtwoHtwoOr and if fuel remains unreacted, other by-products can be produced. For example, him soot They are solid particles of small size, formed mainly by impure carbonwhich occur as a consequence of the incomplete combustion of woodcoal and other hydrocarbon fuels.
The combustion control it is very important in certain applications, for example automobile combustion engines. The device design should ensure adequate airflow that allows a complete combustion and minimize the formation of debris that can damage the device itself.
In addition, the incomplete combustion of hydrocarbons also produces carbon monoxide (CO), a gas with higher environmental impact and more toxic than COtwo.
To get an idea of the higher toxicity, the OSHA permissible exposure limit for CO in air is 50 ppm while it is 5000 ppm for COtwo (ASHRAE recommends not to exceed 1000 ppm). For this reason, the production of carbon monoxide due to incomplete combustion is one of the main safety considerations in home appliancessuch as stoves and cookers.