Modelling of Fly Ash Viscous Deposition and Slagging Prediction of Biomass-Fired Boiler

Abstract

In addressing the ash melting-induced slagging on the high-temperature heating surfaces of biomass-fired boiler, a combined experimental and modelling study of biomass ash viscous deposition has been conducted. A comprehensive ash viscous deposition model based on ash fusion and viscosity characteristics and the critical velocity criterion has been proposed. The deposits of the top furnace and high-temperature superheater of a 130 t/h biomass-fired boiler have been sampled for validation, and a modelling study using ANSYS FLUENT with a user defined function (UDF) and dynamic mesh has been performed. The results show that small particles with diameter of 20 μm lead to less viscous adhesion because of faster response to temperature decrease before reaching the heating surfaces. Deposition efficiency and growth rate of deposits are positively correlated with temperature. Slagging in the top furnace area is dominated by the viscous deposition of molten ash. However, the amount of viscous deposition accounts for a low proportion (20.1%) in the high-temperature superheater, suggesting a change of dominant slagging mechanism from viscous deposition to gaseous condensation and the subsequent capture of fly ash. The model is suitable for predicting ash melting-induced slagging in high-temperature areas of biomass boilers.

Keywords: Biomass-fired boiler, Ash melting-induced slagging, Viscous deposition, Adhesion probability, Slagging prediction.