Calcium hydroxide

Dry calcium hydroxide (Ca(OH)₂) desulfurization process is a technology that removes sulfur dioxide (SO₂) from flue gas through dry absorbent, which is mainly used in flue gas treatment in coal-fired boilers and industrial furnaces. At its core, it produces stable calcium sulfate (CaSO₄) or calcium sulfite (CaSO₃) through the chemical reaction of calcium hydroxide with SO₂. Here's a detailed description of the process:

Process principle

l Main reactions:

n SO₂ absorption:

SO₂+Ca(OH)₂→CaSO₃+H₂O

SO₂+Ca(OH)₂→CaSO₃+H₂O

n Oxidation reactions (if oxygen is involved):

CaSO₃+½O₂→CaSO₄

CaSO₃+½O₂→CaSO₄

n The final product is anhydrous calcium sulfate (gypsum) or calcium sulfite, which needs to be further processed.

l Peculiarity:

The reaction is carried out under dry conditions, without the need for a lot of water, suitable for water-scarce areas, but the desulfurization efficiency (usually 80%-90%) is lower than that of the wet method.

 Process flow

(1) Preparation of absorbents

l Calcium hydroxide is usually used in powder form (about 200 mesh) and can be prepared by the digestion of quicklime (CaO):

CaO+H₂O→Ca(OH)₂

CaO+H₂O→Ca(OH)₂

l Store in a closed compartment to prevent moisture absorption and agglomeration.

(2) Flue gas pretreatment

l The flue gas is first dusted (such as electric dust removal or cloth bag dust removal) to avoid fly ash affecting the reactivity.

(3) Absorption tower reaction

l Injection method:

n Direct powder spraying: Spraying Ca(OH)₂ powder into a flue or a dedicated reactor tower.

l Reaction temperature: controlled at 140-160°C (optimal reaction window).

(4) Particulate matter separation

The reactive flue gas carries a large number of solid particles (unreacted Ca(OH)₂, CaSO₄, fly ash, etc.), which are captured by baghouses or electroprecipitators.

(5) By-product treatment

l The collected ash can:

n landfill (if it contains less unreactants);

n After forced oxidation, it is used as a raw material for building materials (such as gypsum board);

n Part of Ca(OH)₂ can be recycled to improve utilization.

Key influencing factors

l Calcium-sulfur ratio (Ca/S): usually 1.5-2.0, too high to increase the cost, too low to reduce the desulfurization rate.

l Flue gas temperature: too high will cause incomplete reaction, too low may cause condensation blockage.

l Contact time: Longer residence times, such as increasing the height of the reactor tower, can improve efficiency.

l Absorbent activity: Ca(OH)₂ powder with high specific surface area is more effective.