Boiler Feed Water Treatment

Boiler Feed Water

Natural water usually contains solid, liquid and gaseous impurities. It cannot be used for Power Generation.

• Makeup Water is needed  ↠  to replace the losses due to Blow Down and Leakages in the cycle.
• Salt of Calcium and Magnesium is harmful because when heated, it precipitates as solid residue.
• If 1 litre water is added in boiler  ↠  1 - 2 % of water waste as evaporation losses.

Classification of Impurities in Water

Impurities in water are classified into the following types:

A. Visible Impurity

1. Microbiological Growth  ↠  presence of micro-organisms which produce clogging.
2. Turbidity and Sediments  ↠  Turbidity (suspended insoluble matter) and sediments (coarse particles which settle down in stationary water).

B. Dissolved Gases

1. Carbon dioxide
2. Oxygen
3. Nitrogen
4. Methane 
5. Hydrogen Sulphide

 C. Mineral and Salts

1. Iron and Magnesium
2. Sodium and Potassium Salts
3. Flourides
4. Silica

D. Mineral Acids 

Presence of undesirable (reaction with the boiler material).

E. Hardness

Salts of calcium and magnesium (carbonate, chloride, sulphate) are responsible for hardness (which can resist heat transfer and clogs the passage in pipes).

Troubles caused by the Impurities in Water

Problems caused y the impurities in water are:

• Scale  ↣  if precipitate form hard, adhering coating on the inner walls of the boilers.
• Due to salts of Ca and Mg.
• Reduce Heat transfer rate (overheating of tubes and chokes flow)
• Descaling agents are used.
• Sludge  ↣  when precipitation of dissolved salts takes place in the form of loose, non-adherent precipitates.
• Example: CaCO3, CaSO4. 
• Corrosion  ↣  caused by an acid (or low pH) in addition to the presence of dissolved oxygen and CO2 in boiler FW.
• Carry Over  ↣  water solids carried over in the steam leaving a boiler drum.
• Foaming or Priming of boiler water forms the carry over.
• Foaming ↣ bubbles formed on water space due to saponification agent.
• Priming ↣ too much moisture carry over because of high water level or high steaming rate (boiler is filled 60 - 70 %).
• Caustic Embrittlement (or Stress Corrosion Cracking)  ↣  caused due to caustic impurities present in water.
• Presence of concentration of NaOH.
• Due to this, boiler becomes brittle and inner cracks produced.

Methods of Feed Water Treatment

Following are the methods which are used for the treatment of feed water.

1. Mechanical Treatment

The flow chart of mechanical treatment method is:

Untreated Water ↣ Coagulation ↣ Flocculation ↣ Sedimentation ↣ Filtration ↣ Disinfection ↣ Treated water.

• Coagulation  ↣  coagulants like Al2(SO4)3, Na3Al or FeSO4 are added to impure water to remove minute collidal suspensions.
• Flocculation  ↣  it consist of series of steps:
• Increase particle size from sub-microscopic micro-floc to visible suspended particles.
• Micro-flocs  ↣  brought in contact with each other by slow mixing.
• Once floc reached its optimum size and strength ↣ ready for sedimentation.
• Sedimentation  ↣  water is allowed to stand at stand-still in big tanks so solid matters settle down and clean water is drained out.
• Filtration  ↣  raw water is passed through number of membrane (micro-filtration, ultra-filtration, nano-filtration, reverse osmosis) to remove all impurities to get permeate.
• Disinfection  ↣  to remove micro-organisms.

2. Thermal Treatment

It is the removing of dissolved gases and CO2 from feed water.

• Dearation  ↣  used to remove dissolved gases using open type Feedwater heater (called Deaerator).
• Mechanical means for the removal of dissolved gases make use of Henry's Law and Dalton's Law.
• Distillation  ↣  remove salts, sediments, bacteria.
• Heat ↣ Vapor ↣ Condensation ↣ we get pure water as condensate.

3. Chemical Treatment

• Internal Treatment  ↣  regents are added to water in boiler to remove carbonate impurities (like Na2CO3, Na3PO4, NaOH).
• External Treatment  ↣  raw water is added with reagents to remove impurities before entering into boiler.
• It is further divided into three types:
• Lime Soda Softening process
• Hot-Phosphate Softening process 
• Ion Exchange process (Sodium Zeolite or Hydrogen Zeolite process)

a. Lime Soda Softening Process 

It is defined as, "dissolved calcium and magnesium salts removal process using lime Ca(OH)2 and Soda ash Na2CO3".

• Cold Process  ↣  when process is done at normal raw-water temperature.
• Hot Process  ↣  when occur near or at boiling temperature.

b. Hot-Phosphate Softening Process

It is defined as, "dissolved calcium and magnesium hardness removal process using trisodium phosphate Na3PO4 and caustic soda NaOH".

• It also removes silica compounds  ↣  problem with high pressure boiler.
• Carried at or above 100 ℃.

c. Ion Exchange Process

It is defined as, "dissolved calcium and magnesium hardness removal process using solid material of sandy texture (resin or formally called as Zeolite)".

• Raw water  ↣  pass through cation and anion exchanger to remove cations and anions  ↣  clean (or soft) water.
• For anion bed  ↣  Alkaline solution.
• For Cation bed  ↣  Acidic solution.
• Regeneration of Zeolite:
• Back-washing of Bed ↣ pump to regulate water from cation to anion exchanger tank in reverse direction with soda ash.
• Add acid.
• Regenerate the bed.

4. Demineralization

It is defined as, "used to remove all the ions present in water". Steps for demineralization are:

• Water is passed through cation exchanger.
• Then passed through anion exchanger (back-washing using 2% H2SO4).
• Passed through degasifier (de-gas the system using aeration).
• CO2 removal  ↣  Aeration.
• O2 removal  ↣  Dearation.
• Silica Absorber  ↣  adsorbing agent Mg(OH)2 used to stick silica.

Note:

• Industrial Deaerators

References:

• Material from Class Lectures + Book named Power plant Engineering by P.K Nag (4th Edition) + My knowledge. 
• Photoshoped pics are developed. 
• Some pics and GIF from Google.  
• Videos from YouTube (Engineering Sights).