water treatment in power plants

1. Water Treatment

2. Plant Water Requirements
Primary uses:
 Cooling Water
 Service Water
 High purity water

3. MAJOR IMPURITIES OF WATER:

4. • Permanent Hardness: is due to presence of
SO4, NO3, Cl of Ca++ & Mg++
• Temporary Hardness: is due to presence of
HCO3 & CO3 of Ca++ & Mg++ in water
• Alkalinity is of 3 types.
Bicarbonate Alkalinity
Carbonate Alkalinity
hydroxide or caustic Alkalinity

7. Description of Usage
(1)Cooling water includes water used in
condenser for turbine heat rejection and
auxiliary equipment cooling. It can be a once-
through system with discharge returned to river,
a recirculating cooling sys. Using a wet cooling
tower, a closed system employing dry cooling,
or a hybrid system using both wet and dry
cooling for heat rejection.

8. • Once through sys. Require high flow rates but
consume smaller quantities of water. heavily
regulated and permitted only in special
circumstances.
• Cooling tower designs have significant consumption
rates bcoz of high evaporation rates and blowdown
requirements.
• Dry cooling sys. Are relatively expensive and
inefficient.
• Consequently, recirculating cooling sys using a wet
cooling tower are most often used.
• A completely closed cooling water sys is often used
for aux. equipment cooling.

9. • (2)Service water is used for general plant services
such as sanitary water, washdowns, ash transport,
FGD sys makeup and pump seal water. Many of
these services may use reclaimed wastewater.
• A portion of the service water is demineralized to
obtain high purity water for cycle makeup, a
chemical sol. Water, as makeup to closed auxiliary
cooling sys. , and in laboratory.

10. System Requirements
(1)Main steam cycle CW
• For a once through sys it should be ensured that no
suspended particles are present in the water.
• With proper design consideration and material selection
,water with upto approx. 50000 mgL of total dissolved
salts can be used.
• Biological growth is controlled by shock chlorination.
• Mechanical tube cleaning sys can be used to clear marine
growth from tube surface.
• Since suspended sand can erode tubes,cond. Tubes must
be protected by proper intake structure, reasonably low
velocities and selection of materials.
• Ph of once through CW normally ranges bw 6 and 8.5

11. (2)Auxiliary Cooling Water:
• A completely closed loop cooling sys. preferred for oil coolers, air
compressors, bearing water etc.
• This permits use of condensate quality water with corrosion
inhibitors.
• When quality of condensate cooling water is good it may be directly
used for auxiliary cooling.
(3)Service Water:
• Used for pump and instrument seal water, hose supply, fire water,
demineralizer supply, sanitary water and make up to ash and flue gas
scrubbing sys.
• Should be free of suspended solids, turbidity and color.ph b/w 6 and
8.5.
• Preferable to design sys. to meet drinking water standards to avoid
dual piping sys and eliminates necessity of tagging valves and lines as
potable and non potable.

12. (3)High Purity Water:
• Required for makeup to condensate feed water cycle, as solution
water for condensate feed water chemicals.
• To prevent scaling and corrosion and to prevent carryover of solids
with steam from steam generator.
• Higher quality water required for higher operating temperature of
boiler.

13. Treatment Processes
1.Sedimentation & Clarification:
a. Settling:
• Simple settling in large bodies of water in significant detention times.
• Periodic weather conditions and organic growth such as algae
necessitate provisions to ensure clean water at all times.
b. Coagulation:
• Gathering of suspended particles to form larger and more settling
particles.
• Various compounds used: aluminum sulfate(al2(so4)3)[most used],
sodium aluminate(na2al2o4),ferrous sulfate(feso4).
• Activated silica, clays, powdered activated carbon and organic
polymers(polyelectrolytes) are also used to enhance clarification
process.
• Equipment must provide rapid and thorough mixing of coagulant.

14. 2.Aeration:
• It is the process of increasing the oxygen saturation in water
• Water aeration is often required to free the water from dissolved
impurities and this can be achieved by infusion of air at the bottom
or surface agitation.
• It Is done for oxidation of iron, manganese and organic matter.
• The basic two methods of aeration are surface and subsurface.
• Most commonly used methods waterfall aeration and spray aeration.
• Bubble or diffusion method may be accomplished by pumping air into
water through distribution devices such as perforated pipes or porous
plates. Offers high efficiency.
• Mechanical aeration: motor driven impellers that stir surface of water
being treated to increase contact with atmosphere.

15. 3.Softening:
• Removal of Ca and Mg contents by 2 basic methods: chemical
treatment and ion exchange.
• Most commonly used reagents: lime in the form of quicklime(CaO) or
hydrated lime(Ca(OH)2),soda ash(Na2CO3).
• Ion exchange method uses cation type resin.
4.Filtration:
• Separation of suspended and colloidal particles from water by
passage through a porous medium.
• Filter mediums used are sand, anthracite coal, activated carbon, dual
or even multi layered media, fabric or porous membranes.
• Imp of removal of chlorine and organics in demineralization sys
influent water is that both these impurities degrade resin
performance, reducing ion exchange capacity. some reverse osmosis
membranes are also sensitive to chlorine.

16. 4. Demineralization:
It is removal of dissolved ionic impurities that are present in water..
Ion Exchange Processes:
• It is accomplished using resins that exchange one ion for another.
• Cation resins are solid spherical beads with fixed negatively charged
sites and exchangeable positively charged sites.
• Anion resins are solid spherical beads that have fixed positive charge
sites and exchangeable negative charge sites.
• In their regenerated states cation resins are in hydrogen form and
anion resins are in hydroxide form.
• The hydrogen ion(+H) displaced from cation resin reacts with
hydroxide ion displaced from anion resin. Net effect is that dissolved
ions are removed from water and pure water is obtained.
• Ion exchange resins are contained in pressure vessels. The ion
exchange resin in vessels is referred to as resin bed.

17. • When resin bed site is exchanged with a dissolved ion, site becomes
exhausted. Exhausted resins must be regenerated to return resins
beads to original hydrogen form for cations and anions.
• Cation resins are generally regenerated with a strong acid such as
sulfuric or hydrochloric acid. Anion resins are regenerated with
sodium hydroxide sol.
• Dissolved gases such as oxygen and free carbon dioxide are removed
by degasification.