The document discusses various chemical dosing systems used in a power plant. It describes the dosing of ammonia, TSP, carbohydrazide in the HRSG system to control pH, prevent scale and corrosion. It also discusses dosing of corrosion inhibitors, biocides, sulfuric acid and chlorine in the cooling tower and closed cooling water systems. Lime, alum and polyelectrolyte dosing in pretreatment is covered as well for coagulation and flocculation.

1. All chemical dosing system
Md. Arifull Hoque Shaheen

2. HRSG DOSING SYSTEM

3. AMMONIA DOSING SYSTEM
Chemical formula : NH3
Dosing system
Dosing rate :3 ppm

4. Where we dose Ammonia in our plant:-
1.HRSG fill pump discharge
2. CEP( condensate extraction pump) discharge
3.CST (condensate storage tank) discharge.

5. Why we dose ammonia with reaction:-
1. Control pH (9.0-9.8)
2.Neutralize carbonic acid or carbon dioxide.
NH3+H2O=NH4OH
NH4OH+H2CO3=(NH4)2CO3+2H2O

6. TSP DOSING SYSTEM
TSP(TRISODIUM PHOSPHATE)
Chemical formula :Na3PO4

7. Where we dose TSP in our plant:-
1.All boiler drum (HP,IP,LP).
2.Aux. boiler.

8. Why we dose TSP with reaction:-
1.Maintain alkalinity & avoid scale forming in the drum.
2.Avoid deposits of hardness(non detectable) in the boiler (HRSG) tube system.
3.Increase pH value in the drum.

9. How TSP work in drum with reaction:
Na3PO4+H2O=Na2HPO4+NaOH
Na2HPO4+H2O=NaH2PO+NaOH
NaH2PO4+H2O=H3PO4+NaOH
H3PO4+H2O=3H++PO4
+3
In this four step occur in water with TSP and maintain water pH.
The dosing quantities of sodium(18ppm max.) & phosphate(2-6 ppm) are
manually regulated as per results of water sample analysis.Both ratio
Na/PO4= 3.

10. How TSP avoid deposits of hardness in the boiler(HRSG):
TSP react with hardness (If present) then precipitate phosphate as sludge.
Na3PO4+3CaSO4=3NaSO4+Ca3(PO4)2
Na3PO4+3MgSO4=3NaSO4+Mg3(PO4)2
Its very sticky sludge so we need to avoid this by proper maintaining phosphate
dosing.
Need to happen
CaCl2+2NaOH=NaCl+Ca(OH)2
MgCl2+2NaOH=NaCl+Mg(OH)2

11. Carbohydrazide DOSING SYSTEM
It’s another name is Oxygen scavenger.
Chemical formula :CH6N4O
Dosing rate 200 ppb.

12. Where we dose Carbohydrazide in our plant
1.HP/IP BFP suction
2.LP BFP suction.

13. Why we dose Carbohydrazide with reaction
1.Oxygen scavenger (Na2SO3,N2H4)
2.corrosion preventer in boiler surface.

14. How Carbohydrazide work in drum as oxygen
scavenger with reaction
CH6N4O+H2O=2N2H4+CO2 (>135C)
2N2H4+2O2=4H2O+2N2 [Theoretically 1 ppm N2H4 produce 1 ppm CO2.Actualy
1.5 to 2.0 ppm N2H4 produce 1 ppm CO2.]
Decomposition
CH6N4O+H2O=2NH3+N2+H2+CO2 (>200C)
1 ppm carbohydrazide liberates
15 ppb ammonia
14 ppb carbon dioxide.

15. How Carbohydrazide work in drum as corrosion
prevent with reaction
Carbohydrazide also react with Fe2O3(ferric Oxide) in the boiler water to form a
passive magnitude film on the boiler surface preventing from the corrosion.
N2H4+6Fe2O3=4Fe3O4+N2+2H2O
Fe2O3 Fe3O4

16. Cooling tower dosing system
1.Corrosion Inhibitor
2.Bio disparsent
3.Biocide
4.Sulfuric acid.
5.Cl2

17. Corrosion Inhibitor dosing system
Two types corrosion inhibitor
Cathodic inhibitor-zinc,poly/meta/pyro/organic phosphate etc.
Anodic inhibitor-Molybdate,ortho phosphate,nitrite etc
Design basis: dosing level is 20 ppm @ 40% concentration. Or dosing based on
chemical supplier.

18. Why and how much dosing Corrosion inhibitor in CW
Prevent scale formation and possible corrosion.

19. Bio dispersant dosing system
Design basis: dosing level is 20 ppm @ 40% concentration. Or dosing based on
chemical supplier.

20. Why and how much Bio dispersant dosing in CW
Prevent the microbes in dispersion.

21. Biocide dosing system
Design basis: dosage level designed considering the holding volume in cooling
tower basin @ 40 ppm

22. Why and how much Biocide dosing in CW
prevents biological fouling in the cooling water and keeps the microbes in
dispersion.
.

23. Sulfuric acid dosing system
Chemical formula - H2SO4
Design basis: Dosage is designed for a desired alkalinity of 200 ppm in the
cooling water.

24. Why we use Sulfuric acid in CW
adjust the pH and control the scaling.
ABOUT SHOCK DOSING
Once in a every month we give one shock dosing in our CW forebay.
Shock dosing time pH 4-5 (not below 4 and not avob 5). Then 16 hours maintain
this pH and recirculate water through condenser.

25. Chlorine (Cl2)
Based on the dosing of 1.5 ppm in the cooling water flow.

26. How & Why we use Chlorine (Cl2) in CW
Chlorine react with water-
Cl2+H2O=HOCl+HCl
Both are acids formed by hydrolysis react with alkalinity to reduce buffering
capacity of water. 1 pound of chlorine gas added to water removes about 1.4
pound of alkalinity.
HOCl is mainly responsible for killing of bacteria.
We need to regulate Cl2 dosing by maintaining FRC (0.3-0.5).

27. CCW Dosing system
CCW ( Closed Cooling water)
In CCW we dose Two types chemical.
1.Corrosion Inhibitor
2.Biocide

28. Why and how much dosing Corrosion inhibitor in
CCW
Prevent scale formation and possible corrosion. Same work as like CW

29. Why and how much Biocide dosing in CCW
prevents biological fouling in the cooling water and keeps the microbes in
dispersion.
Dosing Day after blowdown.

30. Pretreatment Dosing system

31. 1. Lime(Ca(OH)2) Dosing system
1.Mainly we use lime in clarifier for increase water pH value.
And also for maintain clarity of water.

32. 2. Alum( Al2(SO4)3.18H2O) Dosing system
Uses - 1. For coagulation
Al2(SO4)3.18H2O+3Ca(HCO3)2=2Al(OH)3+3CaSO4+18H2O+6CO2
Al2(SO4)3.18H2O+3Ca(OH)2=2Al(OH)3+3CaSO4+18H2O

33. 3.Polyelectrolyte Dosing system
Uses - 1. For Flocculation

34. 4. Chlorine (Cl2)

35. How & Why we use Chlorine (Cl2) in CW
Chlorine react with water-
Cl2+H2O=HOCl+HCl
Both are acids formed by hydrolysis react with alkalinity to reduce buffering
capacity of water. 1 pound of chlorine gas added to water removes about 1.4
pound of alkalinity.
HOCl is mainly responsible for killing of bacteria.
We need to regulate Cl2 dosing by maintaining FRC (0.3-0.5).

36. Thank you very much