Question

How does limestone react with SO2 to achieve removal of SO2 from exhaust gas stream?

Answer 1

SO2 is an acid gas, and, therefore, the typical sorbent slurries or other materials used to remove the SO2 from the flue gases are alkaline. The reaction taking place in wet scrubbing using a CaCO3 (limestone) slurry produces CaSO3 (calcium sulfite) and can be expressed as:

CaCO3 (solid) + SO2 (gas) → CaSO3 (solid) + CO₂ (gas)

When wet scrubbing with a Ca(OH)₂ (lime) slurry, the reaction also produces CaSO₃ (calcium sulfite) and can be expressed as:

Ca(OH)₂ (solid) + SO₂ (gas) → CaSO₃ (solid) + H₂O (liquid)

When wet scrubbing with a Mg(OH)₂ (magnesium hydroxide) slurry, the reaction produces MgSO₃ (magnesium sulfite) and can be expressed as:

Mg(OH)₂ (solid) + SO₂ (gas) → MgSO₃ (solid) + H₂O (liquid)

To partially offset the cost of the FGD installation, in some designs, the CaSO₃ (calcium sulfite) is further oxidized to produce marketable CaSO₄ · 2H₂O (gypsum). This technique is also known as forced oxidation:

CaSO₃ (solid) + H₂O (liquid) + ½O₂ (gas) → CaSO₄ (solid) + H₂O

A natural alkaline usable to absorb SO₂ is seawater. The SO2 is absorbed in the water, and when oxygen is added reacts to form sulfate ions SO₄- and free H⁺. The surplus of H⁺ is offset by the carbonates in seawater pushing the carbonate equilibrium to release CO₂ gas:

SO₂ (gas) + H₂O (liquid) + ½O₂ (gas)→ SO₄²⁻ (solid) + 2H⁺

HCO₃⁻ + H⁺ → H₂O (liquid) + CO₂ (gas)

In industry a 50 Baume solution of caustic (NaOH) is often used to scrub SO2, producing sodium sulfite:

2NaOH (aq) + SO₂ (gas) → Na₂SO₃ (aq) + H₂O (liquid)

Thus SO2 is removed from exhaust gas stream