Desulfurization Process Explained

Flue gas desulfurization (FGD) is a process used to remove sulfur dioxide (SO₂) from the exhaust flue gases of fossil-fuel power plants, and from other sulfur oxide emitting processes.

Reaction with Slaked Lime

One common method for flue gas desulfurization is wet scrubbing using a sorbent like slaked lime. Slaked lime is calcium hydroxide, chemically represented as Ca(OH)₂.

When SO₂ gas from the flue gas passes through an absorber containing a slurry of slaked lime, it reacts with the slaked lime. The primary reaction producing calcium sulfite is:

$\text{SO}_2\text{(g)} + \text{Ca(OH)}_2\text{(aq)} \rightarrow \text{CaSO}_3\text{(s)} + \text{H}_2\text{O(l)}$

This reaction produces calcium sulfite (CaSO₃) as a solid precipitate and water.

Presence of O₂ and Final Product

The question mentions the presence of O₂ in the process. While the presence of oxygen can lead to the oxidation of calcium sulfite (CaSO₃) to calcium sulfate (CaSO₄), especially in some FGD processes designed to produce gypsum (CaSO₄ ⋅ 2H₂O), the direct reaction between SO₂ and slaked lime primarily yields calcium sulfite.

Calcium sulfite is sparingly soluble in water and tends to crystallize out as hydrates. The degree of hydration can vary depending on the process conditions like temperature and water content. Hydration means that water molecules are incorporated into the crystal structure of the solid.

Based on the options provided, the final product is given as a hydrated form of calcium sulfite.

The formation of calcium sulfite trihydrate involves the calcium sulfite crystal incorporating three water molecules:

$\text{CaSO}_3\text{(s)} + 3\text{H}_2\text{O(l)} \rightarrow \text{CaSO}_3 \sdot 3\text{H}_2\text{O(s)}$

Thus, in the process of desulfurization of flue gas where SO₂ reacts with slaked lime, calcium sulfite is formed, which then hydrates to form calcium sulfite trihydrate (CaSO₃ ⋅ 3H₂O) under specific conditions.