Cement Calcination Emissions

Limestone calcination releases carbon dioxide because heating limestone separates its carbonate component into lime and a gas. Chemically, calcium carbonate (CaCO3) decomposes into calcium oxide (CaO) and CO2; this is a process emission arising from the raw material itself, not from burning fuel.

The chemical reaction

Limestone supplies much of the calcium needed for cement manufacture. When heated sufficiently, its calcium carbonate undergoes thermal decomposition, leaving solid calcium oxide and releasing gaseous carbon dioxide.

• The reaction is CaCO3 + heat → CaO + CO2.
• The reaction requires heat, so energy must be supplied to maintain calcination.
• The carbon in the released CO2 originally belongs to the carbonate mineral; it does not have to come from a fuel.

Calcination within clinker production

Clinker is the intermediate material that is subsequently ground with gypsum to make cement. Calcination prepares the calcium oxide needed for the chemical reactions that form clinker minerals.

• The raw mixture contains limestone and materials supplying silica, alumina and iron oxide.
• In plants equipped with a precalciner, much of the carbonate decomposition occurs before the material enters the rotary kiln.
• At higher temperatures in the kiln, calcium oxide reacts with the other constituents to form clinker minerals, principally calcium silicates.
• The CO2 liberated during calcination leaves with the exhaust gases unless it is captured.

How much carbon dioxide is released?

The quantity follows from the reaction's stoichiometry, meaning the fixed mass relationship between reactants and products.

• Approximately 100 kg of pure CaCO3 produces 56 kg of CaO and 44 kg of CO2 on complete decomposition.
• Thus, complete calcination of one tonne of pure calcium carbonate releases about 0.44 tonnes of CO2.
• This is not an emission factor per tonne of cement: actual emissions depend on the carbonate raw materials used, clinker composition and the proportion of clinker in cement.

Why process emissions need separate mitigation

Process emissions from carbonate decomposition differ from combustion emissions produced by fuels supplying heat. Cleaner energy can reduce the latter but does not, by itself, eliminate CO2 released by the calcination reaction.

• Using less clinker per tonne of cement reduces the associated need for limestone calcination.
• Carbon capture can address CO2 that is still generated during clinker manufacture.