Predicting combustion products of common fuels

Chemistry of the atmosphere • Atmospheric pollutants

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How does incomplete combustion affect energy yield?

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Incomplete combustion releases less energy per mole of fuel because oxidation of carbon is incomplete and some chemical energy remains in CO or soot.

Key concepts

What you'll likely be quizzed about

Complete combustion of hydrocarbons

Hydrocarbons contain carbon and hydrogen atoms. When oxygen supply is ample, carbon atoms oxidise fully to carbon dioxide and hydrogen atoms oxidise to water. Balanced combustion equations show CO2 and H2O as main products, for example: carbon + oxygen → carbon dioxide; hydrogen + oxygen → water. Complete oxidation occurs because sufficient oxygen allows full electron transfer from fuel atoms to oxygen, so no partially oxidised carbon species remain.

Incomplete combustion and limited oxygen

Limited oxygen prevents full oxidation of carbon atoms. Cause: oxygen shortage during burning. Effect: formation of carbon monoxide and carbon particulates (soot) instead of carbon dioxide. Carbon monoxide forms when carbon partially oxidises to CO; soot forms when carbon atoms aggregate without full oxidation. Predict incomplete products when oxygen supply is known to be restricted or flame appears smoky.

Combustion of hydrogen-containing fuels

Fuels that contain hydrogen produce water vapour as a primary product because hydrogen oxidises to H2O. Cause: hydrogen atoms reacting with oxygen. Effect: large amounts of water produced and less CO2 per unit mass for hydrogen-rich fuels. Presence of oxygen within the fuel molecule reduces external oxygen demand and can alter product ratios.

Combustion of sulfur- and nitrogen-containing fuels

Fuels containing sulfur oxidise to sulfur dioxide when oxygen is present. Cause: sulfur atoms oxidise during combustion. Effect: sulfur dioxide appears in exhaust and can form acid rain in the atmosphere. Nitrogen in the fuel or atmospheric nitrogen at high temperatures can form nitrogen oxides (NO and NO2). Cause: high combustion temperatures enable nitrogen and oxygen to react. Effect: NOx formation increases with temperature and contributes to smog and acidification.

Effect of fuel composition and conditions on pollutants

Presence of oxygen, sulfur, nitrogen or impurities in the fuel changes the product mix. Cause: specific atoms within the fuel undergo oxidation to characteristic oxides. Effect: emissions include CO2, H2O, CO, SO2, NOx and particulates depending on composition and oxygen availability. High temperature and excess air favour complete oxidation of carbon but also increase thermal NOx formation. Low temperature and poor mixing favour incomplete combustion and particulates.

Key notes

Important points to keep in mind

Identify all elements in the fuel formula (C, H, S, N, O) before predicting products.

Sufficient oxygen causes carbon→CO2 and hydrogen→H2O; limited oxygen causes CO and soot.

Sulfur in fuel oxidises to SO2; sulfur emissions cause acid rain after atmospheric reactions.

High temperatures and excess air increase thermal NOx formation from atmospheric nitrogen.

Soot and black smoke indicate incomplete combustion and lost energy.

Balanced equations and stoichiometry predict mole ratios of products from reactant amounts.

Internal oxygen in a fuel molecule reduces external oxygen demand and shifts product ratios.

CO is toxic; prediction of CO requires assessment of oxygen availability and mixing quality.

Predict pollutant formation from both fuel composition and combustion conditions (temperature, oxygen, mixing).

Use visual flame clues (colour, smoke) and knowledge of conditions to infer likely combustion products.