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2.3.1. Lime and limestone process 
Lime and limestone scrubbings are non-regenerative wet processes producing gypsum. Lime 
and limestone scrubbing are very similar. The use of lime (CaO) instead of limestone 
(CaCO
3
) for the slurry preparation is the only difference. The alkaline slurry is sprayed in the 
absorber and reacts with the SO
2
in the flue gas. Following chemical reactions occur: 
SO2 dissociation:
SO
2 (gaseous)
→ SO
2 (aqueous)
SO
2
+ H
2
O → H
2
SO
3
H
2
SO
3
→ H
+
+ HSO
3
-
→ 2H
+
+ SO
3
-
Lime or limestone dissolution: 
CaO
(solid)
+ H
2
O → Ca(OH)
2 (aqueous)
→ Ca
2+
+ 2HO
-
or 
CaCO
3 (solid)
+ H
2
O → Ca
2+
+ HCO
3
-
+ HO
-
Reaction between ions: 
Ca
2+
+ SO
3
2-
+ 2H
+
+ 2HO
-
→ CaSO
3 (solid)
+ 2H
2
O 
The following reactions can occur if there is excess oxygen: 
SO
3
2-
+ ½ O
2
→ SO
4
2-
SO
4
2-
+ Ca
2+
→ CaSO
4 (solid) 
Lime and limestone processes are the most popular flue gas desulfurization system for utility 
boilers. Some system has achieved SO2-removal efficiency greater than 95%. Another 
advantage is that these processes produce gypsum, which is saleable. However, these 
processes have limited usage in refineries. 
 

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2.3.2. Dual-alkali scrubbing 
Dual-alkali scrubbing is a non-regenerative process using sodium-based solution and lime or 
limestone to remove SO
2
from flue gases. Following chemical reactions occur: 
Main absorption reactions: 
2NaOH + SO
2
→ Na
2
SO
3
+ H
2
O 
NaOH + SO
2
→ NaHSO
3
Na
2
CO
3
+ SO
2
+ H
2
O → 2NaHSO
3
Na
2
CO
3
+ SO
2
→ Na
2
SO
3
+ CO
2
Na
2
SO
3
+ SO
2
+ H
2
O → 2NaHSO
3
2NaOH + SO
3
→ Na
2
SO
4
+ H
2
O 
2Na
2
SO
3
+ O
2
→ 2Na
2
SO
4 
Regeneration: 
2NaHSO
3
+ Ca(OH)
2
→ Na
2
SO
3
+ CaSO
3
. ½ H
2
O↓ + 
3
/
2 
H
2
O 
Na
2
SO
3
+ Ca(OH)
2
+ ½ H
2
O → 2NaOH + CaSO
3
. ½ H
2
O ↓ 
Na
2
SO
4
+ Ca(OH)
2
→ 2NaOH + CaSO
4
↓ 
This method is attractive because it has a high SO
2 
- removal efficiency and it reduces scaling 
problems. 
2.3.3. Activated char process 
Activated char process is the principal dry process used in refineries. The circulating activated 
char absorbs SO
2
at a temperature comprised between 100 and 200°C. This process has the 
advantage to also eliminate NOx present in the flue gases. The following chemical reactions 
occur: 
Absorption on char and conversion into sulphuric acid: 
SO
2
+ ½ O
2
+ H
2
O → H
2
SO
4
NOx reduction with ammonia: 
4NO + 4NH
3
+ O
2
→ 4N
2
+ 6H
2
O 
Char regeneration at 400°C: 
H
2
SO
4
→ H
2
O + SO
3
2SO
3
+ C → 2SO
2
+ CO
2
After concentration, SO
2
is sent to the Claus unit. 
This process can achieve an SO
2
-removal efficiency of 90 % and a NOx-removal efficiency 
of 70%. 

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