Slagging & Fouling
Two main types of deposits in boilers.
  • Deposits within furnace, in areas directly exposed to flame radiation such as furnace walls and some widely spaced pendant super-heaters.
  • Take place in the hottest parts of boiler
  • Deposits in areas NOT directly exposed to flame radiation such as the more closely spaced tubes in convection sections of boiler
  • Take place as flue gas & suspended fly ash cool down
Effects of deposition on boiler performance
  • Reduction of heat transfer from combustion gas to water-steam
  • Lead to an increase in gas temperature
  • Lead to a further increase in deposition rate
  • Result in continually changing conditions in boiler.
The formation mechanisms of slagging and fouling is very different for both forms of boiler deposits. While indices calculated based on bulk ash properties can be useful for the estimation of a coal propensity to slag or foul, these indices very rarely simulated the mechanisms involved. For an accurate estimation of where and the extend of slagging and fouling requires knowledge of the distribution and size of the minerals species in a coal particle and free minerals coupled with detailed knowledge of the gas flows from the burners through the boiler.  
Common slagging and fouling indices can be used to indicate the like slagging and fouling performance of coals.