ES 639 Physicochemical Processes for Water and Wastewater Treatment Term Paper onPre-treatment of Water for Thermal Power Plants Prashant Adsul (113184002)
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
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
Boiler Water Composition with respect to Operating Pressure
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.
Failed Tubes Showing Signs of Both Corrosion Failure and Overheating
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
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
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.
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
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
Boiler Feed Water Degassing System Membrane Contactor
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.
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