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What is Complete Combustion?

By Kelly Rose
Updated May 21, 2024
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Complete combustion is a chemical reaction in which all of the carbon atoms in a particular substance are entirely consumed. “Combustion” is usually understood to be a synonym for “burning,” though the chemical definition is usually far wider than simply burning with flames or fire. Fires certainly are a form of combustion, and can result in the complete combustion of wood and other matter. There are a number of other possibilities too, though. From a scientific standpoint, this sort of reaction occurs any time oxygen, heat, and any sort of carbon-containing fuel are present together. The carbon atoms bind with the oxygen atoms such that they are exactly paired up, and the heat triggers a conversion — usually to carbon dioxide and water, but this can depend on the chemical composition of the elements at the start point.

If there isn’t enough oxygen in the atmosphere to match every carbon atom, the combustion is usually incomplete, which means that the conversion turns some of the material to gas, but not all of it. The amount of heat required for the reaction in either event usually depends on the fuel, since the temperature needs to be at or above that material’s burning threshold for the reaction to happen at all.

Understanding Combustion Generally

Many materials, and all that sustain life, are considered “carbon based.” Carbon is an element that is nearly ubiquitous, much like oxygen. Every carbon-containing compound has what’s known as an “ignition temperature,” which is the temperature at which it will burn. Different compounds have different thresholds, but heat is always the first requirement. Sometimes this heat can be really low, like that produced from friction when a match strikes a rough surface; in many cases it has to be a lot higher, though.

Once the material is exposed to heat beyond its ignition temperature, the carbon atoms begin to rearrange themselves. They match up with the oxygen in the atmosphere and a small reaction happens on the atomic level that, when viewed from the outside, can often be quite dramatic. Sometimes the whole thing goes up in flames, or it might quickly appear to melt or dissolve; it could go up in smoke and sometimes makes a noise like a bang or a pop. A lot depends on the substances involved, as well as what else the combusting compound contains beyond simple carbon. These secondary elements are usually involved rather by default.

What Makes a Combustion “Complete”

When scientists talk about “complete” combustion, they’re usually discussing an event where everything is consumed by the reaction. In order for this to happen, there needs to be enough available oxygen for every carbon atom in the compound to find a match or a pair in the environmental air. Most of the time this isn’t a problem; the atmosphere usually contains a lot of oxygen. There is usually only a shortage when the reaction happens somewhere more unusual; underground in a cave, for instance, at a very high altitude, or in a lab where the conditions are artificially controlled.

Incomplete Alternatives

The perfect ratio of carbon to oxygen is also referred to as stoichiometric or zero excess air combustion. Combustion can still happen when the ratio is imbalanced, but in these circumstances there is usually something left over — which is to say, the entire compound doesn’t convert to a gas or otherwise change form. In most cases this is known as incomplete combustion. The process is the same up to the end point; the carbon and oxygen match as far as they can, then leave the remainder more or less untouched.

Propane as a Model

Propane combustion serves as an example of a commonly burned hydrocarbon in household use. Usually, propane combustion will occur when the gas in the air mixture is between 2.2 percent and 9.6 percent. This range is referred to as propane’s “limits of flammability.” A properly functioning propane appliance producing an ideal burn will usually give off a blue flame.

Incomplete combustion of propane occurs when the mix ratio is higher or lower than the ideal ratio, but still occurs within the limits of flammability. If the ratio of propane to air is less than the ideal ratio, a “lean burn” will occur, as evidenced by flames that appear to lift away from the burner or go out. Alternatively, a “rich burn” happens when the ratio of propane to air is greater than the ideal ratio, and can be recognized by larger flames that are yellow rather than blue. Incomplete combustion of propane or other hydrocarbons typically will result in carbon monoxide release, an extremely serious environmental and health hazard for humans and most animals. Carbon monoxide is odorless and can’t be detected without special sensors, but is often lethal if inhaled for prolonged periods.

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Discussion Comments
By anon356142 — On Nov 22, 2013

Explain why incomplete combustion happens in car engines.

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