Scaling and Slag Formation in Boilers

Fouling in Boilers

Fouling is a phenomenon where the hot flue gases and the ash precipitate and settle down in the places where the flue gases exit the boiler.

This layer which is formed reduces the gas flow into the Selective Catalytic Reduction tubes.  This can result in poor effluent treatment of the gases.
Fouling is generally removed by soot blowers. 

Slag Formation in Boilers

Slag formation occurs when the temperature of the gases exiting the furnace is above the fusion temperature of the fly ash.  At these temperatures, the fly ash melts and gets deposits on the sides of the furnace.

Slag Formation can lead to problems such as
  • reduced heat transfer from the combustion gases inside the furnace to the water
  • It can lead to further overheating of the boiler gases which, in turn, leads to further deposition.
  • Leads to unpredictable behaviour in the boiler
It is necessary to maintain furnace temperature below the fusion temperature of the ash.  The fusion temperature of the ash can be obtained by testing ash samples at laboratories. 
Slagging can also be prevented or minimized by a Slag Screen Arrangement in Boilers.

Thermal Spray in Boilers

Thermal Spray is a protective coating made on the tubes of the boiler.  The Thermal spray prevents corrosion, damage to the tubes and unscheduled breakdowns.   The material used for coating is usually an alloy.

Alloys based on Iron with added Chromium are used.  Low carbon steel can also be used as a thermal spray as it resembles the weld overlay.  Aluminium based thermal sprays are also used. 

There are different methods of applying the Thermal spray.  The metal is melted by using an electric arc or a gas flame and sprayed on to the tubes.

Scaling in Boilers

Evaporation of water causes the impurities and minerals in the water to concentrate.  Scale Formation in boilers when impurities precipitate from water.  Scaling also occurs when matter which is suspended settles down to the bottom.

Scaling forms usually on heat transfer surfaces.  Scaling affects the heat transfer and thus the overall efficiency of the boiler.  Severe scaling can cause blockages which can be very expensive to remove. 
Some of the contaminants which can form scales are calcium, magnesium, silica.  Calcium and Magnesium form their carbonate and sulphate salts.  These salts get deposited on the tubes and other internal surfaces of the boiler and other equipment. 

Scaling can be prevented by using good demineralized water as feed water.  Effective water treatment and maintaining good water chemistry can prevent scaling to a large extent. 

Silica in Boiler Water

Ordinary Silica is insoluble in water.  But when silica combines with other materials such as lime and soda, it can form scales which are very difficult to remove.  Soda and lime are used in softening units.

Use of silica based lubricants in the thermal plant as well can also result in silica entering the boiler water.  Another source is the presence of unreacted silicon in the feed water. 

If silica is not removed in time, it forms deposits in the turbine nozzles and change the direction of the steam.  The velocities and pressure drops are changed inside the turbine resulting in reduced efficiency.  Uneven nozzle flow can result in torsional vibration due to uneven loading of the blades.  This can result in vibrations. 

Silica deposits in the boiler are difficult to remove.  They equipment has to be dismantled and physically cleaned.  Blasting aluminium oxide on the surface is also a method used in the removal of silica deposits.