The processes of slagging and fouling and the structure of deposits are varialbe due to a number factors. Furnace design, ash content, ash fusibility, ash components, particle size, flow pattern and other factors influence the extent and nature of ash deposition.
To avoid slagging on the furnace wall, the furnace should be able to keep ash particles in suspension and away from furnace surface, distribute heat evenly to avoid localized temperatures, and remove enough heat to achieve appropriate temperatures at the furnace exit that will minimize deposition on convection surface. The furnace should have sufficient wall surface to cool the furnace gas and ash particle to the desired temperature before they reached any superheater surface.
In addition, the furnace also must be correctly proportionated with respect to width, depth and height to limit the potential for ash particle impacting on furnace surfaces. In general, furnaces burning bituminous are smaller than those burning subbituminous and lignite because bitiminous coals have lower slagging potential. However, when the fuel switches from bituminous to subbituminous or lignites, the furnace becomes too small to reduce the gas and particles to the desired temperatures. This is why severe slagging problems occurs when switching coals.
Fusion temperatures provide an indication of the temperature range over which portions of the ash will be a molten fluid or semi-molten, plastic state. High fusion temperatures indicate that ash released in the furnace will cool quickly to a nonsticky state resulting minimal potential for slagging. Conversely, low fusion temperature indicate that ash will remain in a molten or plastic state longer, exposing more of the furnace to potential deposition.
Lignitic ash, which is defined as having more CaO + MgO than Fe2O3, is easier to deposit than bituminous ash, which is defined as haveing more Fe2O3 than the sum of CaO and MgO. Lignitic ash is generally characteristic of low rank coals from westen U.S and is referred western ash. Similarly, bituminous ash is referred as eastern ash.
The alkali metals, sodium and potassium, have long been associated with the fouling tendencies of coal ash. Volatile forms of these elements are vaporized in the furnace at combustion temperature and react with sulfur in the flue gas and other elements in the ash form compounds that deposit on convection surfaces.
The base to acid ratio of ash also affect the slagging potential. The basic constituents are iron, the alkaline earth metals calcium and magnesium, and the alkali metals sodium and potassium. Acidic constituents are silicon, aluminum and titanium. Ash that is with either high or low base to acid ratio generally has high ash fusion and melting temperatures. However, ash with medium base to acid ration tend to have low fusion and melting temperatures.