Application of chemical equilibrium calculations for the prediction of ash-induced agglomeration

Abstract

Energy crops and biogeneous residues offer the highest potential for future growth in biomass utilization. Traditional forest-grown wood types, along with their consistent combustion characteristics, will thus be replaced by fuels with highly heterogeneous composition. Reliable prediction of their combustion characteristics and in particular of their ash behavior is essential for plant designers and operators trying to harvest this potential for energy conversion. In fluidized bed combustion, the fuel ash-induced agglomeration of the bed materials is one such behavior that needs to be described. This paper describes the application of chemical equilibrium calculations to determine the governing properties of the agglomeration process. Using FactSage 6.2, the thickness of ash coatings and their resulting compositions were evaluated. These were then used to deduce properties of the molten ash (viscosity, surface tension, and density), from adapted empirical models. The validation of the properties was performed on experimental data published in literature. Calculated coating thickness shows good agreement with microscopic measurements of agglomerated bed material. The adapted model for surface tension is within 10% of published results. The viscosity model published by Lakatos was chosen to give the best agreement for biomass ash-derived melts in the applicable temperature region. The results can thus be readily used for analytical modeling or numerical approaches aimed at determining agglomeration tendencies of novel biomass species.