An Integrated Slagging Model of Biomass-Fired Boiler Combining Condensation Fouling and Ash Viscous Deposition

Abstract

Slagging, fouling and ash depositon issues occurring on the heating surfaces of biomass-fired boiler inhibit the biomass firing/co-firing utilization. Fouling on the heating surfaces caused by condensation of salt vapor in the flue gas can also enhance the ash deposition by forming a viscous initial layer, which further captures fly ash. For multiple slagging prediction, an integrated model considering ash direct deposition, gaseous condensation and the subsequent ash capture has been proposed. Simulations of slagging in the superheater area were thus performed using ANSYS FLUENT with user define function (UDF), which were validated by sampling results of experimental study. The results show that the inertial impaction dominates the deposition of smaller particles (10-30 μm) with higher critical velocity, and the viscous capture behavior has obvious effect on larger particles of 50 μm and 80 μm. Condensation is inhibited with increasing temperature of deposited surface, while deposition efficiency increases due to a higher deposited surface viscosity. The proportion of viscous capture in the total deposition mass stays almost unchanged under different wall temperatures in three cases, with an average value of 26.9 %. The model is suitable for predicting multiple slagging behavior in the medium-temperature superheater area of biomass-fired boiler.

Keywords: Biomass-fired boiler, Gaseous condensation, Ash deposition, Viscous Capture, Slagging prediction