Thermal power plants require ultra pure water known as boiler feedwater that is pretreated to remove impurities which can cause scaling, corrosion, and other problems. Common pretreatment methods include filtration, coagulation, ion exchange, chemical precipitation, and deaeration to remove suspended solids, dissolved gases, and minerals. Emerging technologies like membrane contactors and reverse osmosis are also being used to further purify boiler feedwater from wastewater and other sources. Proper pretreatment is crucial to minimize damage to high pressure boilers in thermal power plants.

1. ES 639
Physicochemical Processes for
Water and Wastewater Treatment
Term Paper on
Pre-treatment of Water for
Thermal Power Plants
Prashant Adsul (113184002)

2. Water for Thermal Power Plants
• Thermal power plants contribute 65.09 % to
the total power generation of India
• High pressure boilers are used for steam
generation in thermal power plants
• Ultra pure water is required for the steam
generation, it is called as Boiler Feedwater
(BFW)
• BFW is 95 % condensate and 5 % makeup
water

3. Water for Boilers
• All natural waters contain varying amounts of
suspended and dissolved solids as well as
dissolved gases (O2, CO2)
• The type and amount of impurities in fresh
water vary with the source (lake, river, well)
• Impurities in water are of importance when
water is to be used for steam generation.
• For higher-pressure boilers, feedwater must
be pretreated to remove impurities

4. Boiler Water Composition with respect to
Operating Pressure

5. Impurities in Water and its Effect on
Boiler

6. Problems Caused by Impurities in the
Feed Water
• Scaling
• Water Hardness is the primary source of scale in boiler
equipment, Calcium and Magnesium are main cause of
hardness in water
• Silica in boiler feedwater can also cause hard dense scale with
a high resistance to heat transfer.

7. Boiler Scale on Water Side

8. Failed Tubes Showing Signs of Both Corrosion
Failure and Overheating

9. Corrosion
• Oxygen and Carbon Dioxide cause corrosion
• The carbon dioxide by dissolving in the water
and forming a weak carbonic acid which
attacks the metal in boiler
• Oxygen is present in water reacts with Iron to
form red iron oxide
• Corrosion reduces the thickness of the steam
pipe metal

10. Boiler Feedwater Treatment
• Removing impurities from boiler feedwater
• Feedwater is filtered to remove suspended matter and if the
suspended solids are very fine, a flocculation step may be
needed to enable effective filtration
• Filtration
• Filtration is the essential first step before the chemical
treatment. Filtration minimizes suspended solid impurities. If
rust, sand (silica) etc. are not filtered out, they lead to severe
scale formation.
• Coagulation and flocculation

11. Boiler Feedwater Treatment
• Chemical precipitation
• Chemical precipitation is a process in which chemical added
reacts with dissolved minerals in the water to produce a
relatively insoluble reaction product. Precipitation methods
are used in reducing dissolved hardness, alkalinity, and silica.
The most common example is lime-soda treatment
• Ion Exchange
• Passing water through a simple cation exchange softener all
the Calcium and Magnesium ions are removed and replaced
with Sodium ions.

12. Boiler Feedwater Treatment
• Deaeration of water
Dissolved oxygen in water is a major cause of boiler system
corrosion. It should be removed before the water is put in the
boiler. Feedwater deaeration removes oxygen by
• Heating the water with steam in a deaerating heater. Part of
the steam is vented, carrying with it the bulk of the dissolved
oxygen.
• Oxygen Scavengers
Corrosion by oxygen in the boiler can be controlled by the
addition of Sodium Sulfite (Na2SO3) and Hydrazine (N2H4) to
the preboiler section of the steam generating system.
N2H4 + O2 → 2 H2O + N2

13. Deaeration of water
Membrane Contactors
• Fabricated with hydrophobic hollow
fiber microporous membrane
• Installed to control corrosion in boilers
• By removing gases such as oxygen and
Carbon dioxide the amount of corrosion is
reduced
• These devices do not require chemicals to
operate so operating costs are less

14. Boiler Feed Water Degassing System
Membrane Contactor

15. Membrane Contactor

16. Boiler Feedwater Treatment
• Reverse Osmosis
in combination with other advanced technologies, RO is being
used to treat brackish water, industrial wastewater for Boiler
feedwater pretreatment.

17. REFERENCES
• Chai Hoon Koo, Abdul Wahab Mohammad, Fatihah Suja., (2011).Recycling
of Oleochemical Wastewater for Boiler Feed Water using Reverse Osmosis
Membranes - A Case Study. Desalination, 271, 178–186
• Cuda P., Petr P., Tenglerova J.,(2006) Reverse Osmosis in Water Treatment
for Boilers. Desalination 198 (2006) 41–46
• http://www.cip.ukcentre.com
• http://www.lenntech.com
• http://www.water-technology.net
• Nooijen W.F.J.M., Boks P., Vaal P.R. and Suratt W. B.,(1998). Production of
Boiler Feed Water out of Wastewater with Microfiltration and Reverse
Osmosis: The New Age Challenge within Reach. Desalination, 118, 263–
265
• Shao J., Liu H., He Y.,(2008). Boiler Feed Water Deoxygenation using
Hollow Fiber Membrane Contactor. Desalination 234, 370–377