Power Plant Chemical Reaction

The combined cycle power plant the is burn nature gas to generate electricity, in the process there is lot of chemical reaction involved to the entire production process, to understand that chemical reaction process is very important for the power engineer working on the power plant, this paper will introduce this kind of chemical reaction:

 

1, Nature gas combusted in the gas turbine

The main component of natural gas is the hydrocarbon called methane (CH4). The combustion of methane is represented by the equation:

CH₄ + 2O₂ → CO₂ + 2H₂O + Heat

In the meaning time due to high temperatures, they undergo an endothermic reaction producing various oxides of nitrogen(NOx)

 

2, SCR for NOx reduction 

Gas turbine exhaust gas path through the HRSG heating surface to release the heating, before release from stack, using ammonia to decrease NOx emission, the reaction is:

4NO + 4NH₃ + O₂ → 4N₂ + 6H₂O

2NO₂ + 4NH₃ + O₂ → 3N₂ + 6H₂O

NO + NO₂ + 2NH₃ → 2N₂ + 3H₂O

 

3, Boiler passivation 

To prevent the flow accelerated corrosion (FAC), the HRSG drum, economizer, superheater and feed water pipe steel surface is passivated to forma magnetism film, the reaction is:

3Fe + H₂O → Fe₃O₄+  H₂

4, Boiler oxygen corrosion 

Unfortunately, the corrosion will happen inside the metal surface if the protection film damaged and too much oxygen inside of the water, the corrosion process reaction equation:

2Fe+H₂O +O₂ → Fe₂O₃+H2

5, Use Amine Increase Condensate water PH

Higher PH in the boiler drum, the less chance of corrosion happens inside of the boiler, so to add amine which is all-validation of material to increase the PH:

R-NH₂ + H₂O →  R-NH₃⁺ + OH-

R-NH₃  + OH^– +  H₂CO₃  →  R-NH₃ + HCO₃ + H₂O

This program became known as all-volatile treatment reducing [AVT(R)].

 

6, Oxygen scavenging 

Older plant to add the oxygen scavenger to the condensate line to control the oxygen to prevent the corrosion the reaction is:

Na₂SO₃  + O₂  →  Na₂SO₄

N₂H₂  +  O₂   →  N₂  +  2H₂O

The process is using a carbon bed catalyst

 

7, Phosphate for boiler water treatment program

Control the HRSG drum water deposition and prevent sludge build in the drum, also can adjust  the PH for the drum water, so the phosphate to add to inside of drum, the equation is:

NaH₂PO₄ + H₂O  → HPO₄^2-  + H+  + Na+  +H2O

Na₂HPO₄  +  H₂O  → HPO4^2-  + 2Na+  +H₂O

Na₃PO₄  +H₂O →  HPO₄^2-  +  2Na+   OH^–

Na₂HPO₄  +  NaOH  →  Na₃PO₄  +  H₂O

 

8, Cooling tower dichlorination 

Due to large amount of organically material and biology reaction, free chlorine is most popular  biocide, hypochlorous acid can kill the bugs in the water.

Cl₂  + H₂O  →   HOCl + HCl

NaOCl + H₂O  → HOCl + NaOH

Sodium hypochlorite + water →  hypochlorous acid + water

When the PH increase the hypochlorous acid is decomposed as below:

HOCl   →  H⁺  +  OCl^–

 

9, RO dichlorination 

For the RO membrane, the free chlorine can burn the membrane, so it is requests to dichlorination, to add the sodium bisulphite to dechlorane, the equitation is:

NaHSO₃ + Cl_{2    →}     NaSO₄  +     HCl

2NaHSO₃  + 2HOCl  _→  H₂SO₄ + 2HCl + Na₂SO₄

10, RO Caustic injection

Due to the carbine dioxide is gas will free to pass RO membrane, so inject caustic for neutralization then reject by second pass membrane to produce high quality water:

CO₂+H₂O  → H₂CO₃

H₂CO₃ + NaOH → Na₂CO₃+ H₂O

 

11, Ion exchange resin

Power plant thermal cycle process request make up high quality water, using cation and anion and mixed bed to provide high quality water which is process is:

Cation remove:

2 RNa + Ca⁺⁺  →   R₂Ca + 2 Na+

2 R’H + Ca⁺⁺ → R₂Ca + 2 H⁺

R’H + Na⁺ → R’Na + H⁺

R’H: cation resin

Anion remove:

R’’OH + Cl^–  →  R’’Cl + OH^–

2 R’’OH + SO₄ → R’’₂SO₄ + 2 OH^–

R’’OH: anion resin

12, Resin regeneration:

Exhaust resin can be regeneration through the below process :

R₂Ca + 2 NaCl → 2 RNa + CaCl₂

R’Na + HCl → R’H + NaCl

R’’Cl + NaOH → R’’OH + NaCl

Conclusion:

Above is the major Combined power plant chemistry equation, you maybe found more, but it is very interesting put it together to help understand power plant water chemistry.