Why cement plants release so much CO2
Thermal decomposition of calcium carbonate · Decomposition · Endothermic
Even with perfectly clean fuel, making cement releases CO2, because the gas comes out of the rock itself.
Reactants approach
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ΔH = +178 kJ · endothermic, absorbs heat
ΔH per mol CaCO3.
Meet the molecules
- Reactantsolid
CaCO3
calcium carbonate (limestone)
- Productsolid
CaO
calcium oxide (quicklime)
- Productgas
CO2
carbon dioxide
What you notice
Concrete is the most-used material on Earth after water, and making cement, the binder that holds it together, causes about 8% of global CO2 emissions. Roughly 60% of that comes not from fuel but from the limestone itself. In a cement kiln, crushed limestone is heated above about 900 °C, driving off carbon dioxide and leaving quicklime, calcium oxide, which then reacts with clay minerals at around 1450 °C to form cement clinker. Each tonne of limestone gives off about 440 kg of CO2, however clean the fuel is.
Safety: Quicklime reacts with water violently enough to boil it and causes severe burns to skin and eyes.
Step by step
- 1
Heat goes in
Breaking up the carbonate ion takes energy: 178 kJ per mole (100 g) of CaCO3, supplied by burning fuel.
- 2
Carbonate splits
Each CO3^2– ion gives up a CO2 molecule, leaving an O^2– ion that stays with Ca2+ as CaO.
- 3
Entropy sets the temperature
ΔH is positive, but releasing a gas makes ΔS large and positive. Above about 840 °C (with CO2 at 1 atm), TΔS outweighs ΔH and ΔG becomes negative.
- 4
CO2 escapes
The gas is carried up the chimney, so the reaction runs to completion instead of settling at equilibrium.