Incomplete combustion

This means that the combustion reaction has not gone as far as it could in terms of releasing energy. Not all of the reactant has thoroughly reacted and, in the case of an organic molecule, not all of the carbon has turned to carbon dioxide.

As hydrocarbons are used for fuels, incomplete combustion is both inefficient and dirty, in the sense that the products may be solid carbon (smoke) or carbon monoxide, CO, a harmful odourless colourless gas.

Note: all of the hydrogen contained in a fuel produces water - there are no exceptions.

Fuels that burn efficiently do not release much visible light during combustion - the flame may even appear virtually invisible. This is the case for combustion of the lower relative mass alcohols such as methanol and ethanol in plentiful air.

If the air supply is restricted, for example by using a spirit burner with alcohol, then some of the fuel burns inefficiently forming carbon microparticles that glow in the heat of the flame giving the flame a yellow colour.

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Particulate Matter

When hydrocarbon fuels burn incompletely, the carbon that is produced makes tiny particles of solid carbon suspended in the air. These tiny particles of carbon are called microparticulates. They can be any size and are known to be harmful to the lungs where they act as foci for the formation of localised damage and possible lung cancer.

When the particles are large, we can see them as smoke, which normally settles out of the air onto the surface. However, smaller microparticulates may remain suspended in air for a very long time. Most governments now monitor these materials using a crude measurement system of PM₁₀, PM_2.5 etc. To indicate microparticular level with particles that measure on the micrometre scale.

PM₁₀ measures all of those particles with a diameter of 10 micrometres or less, 1 x 10^-5m.

Worldwide, exposure to PM_2.5 contributed to 4.1 million deaths from heart disease, stroke, lung cancer, chronic lung disease, and respiratory infections in 2016. Overall, ambient particulate matter is one of the leading risk factor for premature death globally. [ref Wikipedia]

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Carbon monoxide

Carbon monoxide is formed when hydrocarbon fuels are burned in insufficient air for complete combustion. The gas is colourless, odourless and highly toxic.

Carbon monoxide is a good ligand for transition metal ions, forming a coordinate bond to the ion using the lone pair of electrons on the carbon atom.

Haemoglobin has an iron atom at the centre of a porphyrin structure that is used to capture oxygen from the lungs and transport it around the body in the bloodstream.

Carbon monoxide coordinates to this iron atom in the +2 oxidation state) forming carboxyhaemoglobin, which renders the haemoglobin incapable of oxygen transport. The brain becomes starved of oxygen resulting in unconsciousness and death.

Incomplete combustion is the reason why all hydrocarbon fuel domestic boilers must be checked on a regular basis to ensure that the air flow is sufficient to prevent the build up of deadly carbon monoxide gases in the home.

Tragedy in Valencia

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Equations for incomplete combustion

There are no specific equations for incomplete combustion, as there are many ratios of reactants from which products can be formed.

Taking the simplest hydrocarbon, methane, we can propose several possible incomplete combustion equations, simply by varying the ratio of methane to oxygen.

Complete combustion of methane

CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(l)

Remember that all of the hydrogen MUST become water, so we need at least one molecule of oxygen

If there is less oxygen than is needed for complete combustion then carbon monoxide and/or carbon is formed. Any ratio of methane to oxygen can be selected for incomplete combustion providing that the amount of oxygen is less than that required for complete combustion.

Incomplete combustion of methane

2CH₄(g) + 3O₂(g) → 2CO(g) + 4H₂O(l)

Another possible equation:

Incomplete combustion of methane

2CH₄(g) + 3O₂(g) → C(s) + CO₂(g) + 4H₂O(l)

It is the same story for every other fuel.

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Pollution from cars

Internal combustion engines mix air with petrol vapour and explode it using a spark inside a restricted space, a cylinder. The resultant explosion pushes the piston back up the cylinder and causes the crank shaft to rotate, driving the car wheels via gears.

The petrol-air mixture must be just right for a powerful explosion, while reducing the amount of incomplete combustion products. The petrol and air are mixed together before injection into each cylinder.

Petrol is mostly 2,2,4-trimethylpentane, C₈H₁₈, which burns incompletely at the most favourable ratio forming both carbon monoxide and carbon dioxide (as well as water).

A possible equation for the combustion of petrol

C₈H₁₈(g) + 12O₂(g) → CO(g) + 7CO₂(g) + 9H₂O(l)

There are millions of cars burning petrol in the world using internal combustion, meaning that carbon monoxide is a common pollutant gas, particularly in cities.

Car manufacturers incorporate catalytic converter in the car exhaust system to reduce this carbon monoxide pollution.

The catalytic converter makes use of another pollutant gas produced in the engine, nitrogen monoxide, NO, and uses a platinum/palladium/rhodium alloy to catalyse the reaction:

Catalytic converter reaction

2CO(g) + 2NO(g) → 2CO₂(g) + N₂(g)

The catalytic converter helps to reduce the amount of these two pollutant gases. However, the convertion is not 100% efficient and also unburnt hydrocarbons and microparticulates are released via the exhaust, making cars a major pollution source in cities.

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