Slagging and Fouling
Based on different mechanisms involved in ash deposit on the heat surface, two general types of ash deposition have been defined as slagging and fouling. Slagging is the formation of molten or partially fused deposits on furnace walls or convection surfaces exposed to radiant heat. Fouling is defined as the formation of deposit on convection heat surfaces such as superheater and reheaters.
Slag formed when melt or soften ash particles is not cooled down to solid state when they reach the heat surface. Typical initial deformation temperatures (or so called “ash fusion” temperatures) are 2100 to 2200 F. With furnace gas temperatures in excess of 2500 F, we have semi-molten ash that may stick to the relatively cooler walls. Most ash tend to be resolidified due to the raltaively lower temperature at the tube surface and the particles factures on impact and partially bounce back into the gas stream. However, if the furnace is too small, the exit gas temperature is too high or the melting point of ash is relatively low, molten ash may not have enough time to be resolidified when they hit the heat surface, therefore they are easy to stick to the surface and cause the accumulation of deposits leading to slagging. Depending on the strength and physical characteristic of the deposit, steam or air sootblower may be able to remove most of them. However, the base deposit generally remains attached to the tube, allowing subsequent deposits to accumulate much more rapidly.
Fouling is generally caused by the vaporization of volatile inorganic elements in the coal during combustion. When heat is aborbed and temperatures decrease in the convection area of the boiler, compounds formed by these elements condense on ash particles and heating surface, forming a glue which initiate deposition