How combustion produces common atmospheric pollutants

Chemistry of the atmosphere • Atmospheric pollutants

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How does combustion temperature affect NOx formation?

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Higher combustion temperatures increase reaction rates between nitrogen and oxygen and therefore increase thermal NOx formation.

Key concepts

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Carbon monoxide (CO): formation by incomplete combustion

Carbon monoxide forms when carbon-containing fuels burn with insufficient oxygen for complete combustion. Limited oxygen supply prevents full oxidation of carbon to carbon dioxide and causes partial oxidation to carbon monoxide. Poor mixing of fuel and air, clogged burners or malfunctioning engines create the low-oxygen conditions that favour CO production. Temperature affects reaction rates, but oxygen availability is the main limiting factor for CO formation.

Soot (carbon particles): formation by incomplete combustion and condensation

Soot consists of fine solid carbon particles produced when hydrocarbon fuels burn incompletely. Very rich fuel mixtures, low oxygen supply or rapid cooling of hot combustion gases cause incomplete oxidation of hydrocarbons and lead to particle formation. High-molecular-weight fragments from broken hydrocarbon chains condense and agglomerate into soot particles. Soot production increases with higher fuel-to-air ratios and poor combustion efficiency and decreases when combustion is hotter and better mixed with oxygen.

Sulfur dioxide (SO2): oxidation of sulfur impurities in fuels

Sulfur dioxide forms when sulfur present in fossil fuels oxidises during combustion. Sulfur atoms in coal, oil or diesel react with oxygen to form SO2 at combustion temperatures. The amount of SO2 produced depends on the sulfur content of the fuel; low-sulfur fuels yield less SO2. Combustion temperature and oxygen availability allow the oxidation reaction to proceed, while post-combustion processes or flue-gas desulfurisation can limit SO2 emissions.

Oxides of nitrogen (NO and NO2): thermal and fuel NOx formation

Oxides of nitrogen form by two main routes during combustion. Thermal NOx forms when atmospheric nitrogen reacts with oxygen at high temperatures in the flame; higher temperatures increase the rate of N2 + O2 reactions to produce NO. Fuel NOx forms when nitrogen contained in the fuel oxidises during combustion. Rapid cooling and good combustion control reduce thermal NOx, while fuel choice and pre-treatment affect fuel NOx. Oxidation of NO to NO2 occurs further in the exhaust gases and in the atmosphere.

Key notes

Important points to keep in mind

Incomplete combustion causes CO and soot formation; oxygen availability is the key limiting factor.

Carbon monoxide is a colourless, poisonous gas produced by partial oxidation of carbon.

Soot consists of solid carbon particles formed by condensation of heavy hydrocarbon fragments.

Sulfur dioxide forms when sulfur impurities oxidise during combustion; fuel sulfur content controls quantity.

Oxides of nitrogen form by high-temperature reactions of nitrogen and oxygen and from nitrogen in the fuel.

Higher flame temperatures increase thermal NOx formation; better combustion control reduces NOx.

Post-combustion treatments (e.g., flue-gas desulfurisation, catalytic converters) limit pollutant emissions.

Fuel choice, burner condition and air–fuel mixing determine the relative amounts of CO, soot, SO2 and NOx.