Slagging of ash from sub-bituminous and bituminous coals in a test combustion facility

Slagging caused by deposit of molten fly ash on hot walls is a major concern in the operation of full-scale utility boilers. We carried out a comprehensive study, experimental and modeling, on slagging with various coals. Coal samples were taken prior and during combustion and analyzed by SEM (Scanning Electron Microscope). From the SEM analysis the coals could be divided into two types: (1) Coal with tiny particulates of the mineral matter deposited loosely on the surface of coal particles or between carbon particles (external ash) and (2) coal with the mineral matter encapsulated within the coal particles (internal ash). We found different slagging and char combustion characteristics directly related to the two coal types. It was observed that internal ash coals show higher slagging propensity and higher carbon content in the fly ash. Previous models to predict slagging did not distinguish between the two coal types and their impact on slagging and combustion behavior. We developed a model for high temperature ash deposition on the furnace walls for these two coal types. In this model, char combustion and carbon content in the fly ash are also considered. Comparison of experimental observation with calculation results from the ash deposition model show good agreement. Another conclusion from the model is that slagging propensity for internal ash coals increases with coal particle size. However, this conclusion has to be verified experimentally.