Slagging and Fouling in Coal Fired Boilers

A large percentage of boilers in both power stations and process industry use coal as a fuel for steam generation. The age of the coal can vary from lignite to anthracite. The inorganics in coal can vary in quantity and in its characteristics. This will alter the type of deposits and also the slagging propensity in boilers. Coal ash property is one of the factors that can cause slagging in boilers. The operating and design conditions also have an impact on deposition and slagging in boiler furnace as well as in the convection pass. The quantity of ash in coal will have a direct impact on the severity of both deposition and slagging.

The molten or partial melting pasty ash deposits in radiant heat transfer surface like boiler furnace is termed as slagging. The sintered ash deposit in the convection region in boiler is called fouling. Both have their own impact on boiler performance like reduction in operating efficiency, availability, higher load on bottom ash handling, etc.

For any deposition to take place there are three requirements. The first one being the ash and the volatiles will have to penetrate into the boundary layer of the tube and make contact with the tube surface. The second is the material must adhere to the tube surface. Third, sufficient cohesion must occur in the deposit for allowing continued growth such that periodic shredding does not happen on its own weight. In a coal fired boiler the first layer of deposit will be rich in sodium, generally sodium sulphate, which initiates fouling. The second layer normally will have weakly bonded discrete particles and will form on the upstream side of the tube. The third layer is the buck portion of the deposit and can be semi continuous.

The following will have to be taken into account when we are dealing with deposition or slagging in coal fired boilers.

  • Coal organic properties
  • Coal mineral matter properties
  • Combustion kinetics
  • Mineral transformation and decomposition
  • Fluid dynamics
  • Ash transport phenomena
  • Vaporization and condensation of ash species
  • Deposition chemistry-specie migration and reaction
  • Heat transfer to and from the deposit
  • Crystallization of coal ash melts
  • Ash composition
  • Ash properties
  • Ash transformation and decomposition
  • Vaporization and condensation of ash species
  • Crystallization of coal ash melts
  • Coal proximate analysis
  • HHV of coal
  • Petrographic analysis/properties.

There are many correlations that are used to predict slagging based on the coal property. A matrix of all these correlations gives fairly a good idea about slagging characteristics of a coal. But this is generally true for coals formed by insitu theory of coal formation and not the Gondvana coals. Lot of studies has been carried out in boilers and coal to understand slagging better. However it is still not possible to predict slagging in coal fired boilers, and designers only use a conservative design to contain slagging.

About the Author

Dr V T Sathyanathan is a boiler consultant with 35 years of experience in various areas of high pressure boiler trouble shooting. He holds a PhD in coal combustion in boilers.

Related Reading

Coal Formation Theories – Coal may be defined as a compact stratified mass of plant debris which has been modified chemically and physically by natural agencies, interspersed with smaller amounts of inorganic matter. In situ and Drift are the two major theories of coal formation.