Prediction of ash deposition on superheater tubes from pulverized coal combustion

A numerical model has been developed to describe the main deposition processes of ash particles with a predetermined chemical composition. Thermophoresis and inertial impaction are considered, the latter being the predominant process for particles > 10 mu m. A single drop-tube furnace and a horizontal p.f.-fired furnace have been modelled in this study. Predicted results show that the denser (iron-rich) particles are more inclined to deposit than less dense (silica-rich) particles. Boiler aerodynamics are also shown to be a controlling factor in the slagging process. Ash layer formation is shown to enhance the deposition process further owing to the increased surface temperature of the deposit. Inclusion of a variable deposit temperature results in good agreement with experimentally measured values. The effect of gas temperature is shown to influence the deposition process owing to the effects on particle viscosity. Predicted results show good agreement with both laminar drop tube and p.f.-fired furnaces for fouling coals, but it has not yet been possible to model non-fouling coals accurately, since ash erosion and deposit breakage have not been considered.