• Fouling occurs due to condensation of inorganic material found in coal.glowing coal image by Georgios Alexandris from Fotolia.com
  
  Fouling occurs in a boiler's convection heat transfer surface as a result of the presence of inorganic material in the fuel. This is caused by condensation from vaporized inorganic materials and usually happens as the gas temperature decreases. Fouling may affect heat transfer and impede the efficiency of a boiler unit.
  
  

Low-Melting Matter

• The main cause of fouling in a coal-fired boiler is the presence of large amounts of low-melting materials. The low-melting constituents that are typically present in coal include NaCl, Na2SO4, CaCl2, MgCl2 and Al2(SO4)3. The melting point of these materials ranges from 700 to 850 degrees Celcius.
  
  

Fouling Deposits

• Fouling deposits in coal-fired boilers form in two layers. The first layer forms as a result of vapors passing through the boiler's heating elements. The vapors leave behind residue that provides a surface for the second layer of residuals to adhere to. The second layer consists of friable deposits and makes up the bulk of the material responsible for fouling. This material binds to the back side of the heating tube as it relates to the direction of gas flow through the unit.
  
  

Ash Composition

• In coal-fired boilers, the composition of the ash produced by the combustion of the fuel may contribute to fouling. The convection heat transfer surface may become contaminated by the build up of ash particles, which may contain different melting temperatures. As the ash is produced, its composition may allow for certain deposits to form within the furnace.