Double alkali method (Na₂CO₃／Ca(OH)₂) in lime method based on the combination of sodium alkali, the salt soluble in water, in the absorption tower with sodium alkali absorption SO₂, desulfurization liquid absorption after regeneration tank using cheap lime for regeneration, which makes use of sodium ion absorption cycle.The process combines the characteristics of lime method and sodium alkali method, solves the problem of easy scaling in the tower of lime method, and has the advantages of high sodium alkali absorption efficiency.

Double alkali method is widely used in medium and small boilers for flue gas desulfurization technology, which is developed to overcome the shortcomings of limestone lime method. There are many kinds of double alkali method, the most commonly used is sodium calcium double alkali method.

Sodium calcium dual alkali (Na₂CO₃--Ca(OH)₂) using soda absorption SO₂, lime regeneration, regeneration after absorbent recycling, no waste water discharge.

Chemical reaction principle

The absorption of SO₂ in the tower

Na₂CO₃＋SO₂＝Na₂SO₃＋CO₂ （1）

2NaOH＋SO₂＝Na₂SO₃+H₂O  （2）

Na₂SO₃＋SO₂＋H₂O＝2NaHSO₃  （3）

Sodium alkali regeneration

Ca（OH）₂ ＋Na₂SO₃＝2NaOH＋CaSO₃

Ca（OH）₂ ＋2NaHSO₃＝Na₂SO₃＋CaSO₃• H₂O＋1 H₂O

In the lime slurry (of lime to saturated condition), NaHSO₃ and Ca（OH）₂ quickly response to release (Na +), [SO₃₂ -] and [Ca₂ +] reaction, reaction generated CaSO₃ settle with hemihydrate form so that [Na +] get rebirth. Na₂CO₃ is just a start, actually consumed after starting is lime, theoretically no consumption of soda ash (just slag removal will take some, too, so a small loss)