Evaluation of agglomeration of biomass ash during fluidized-bed combustion

Biomass is considered as one of the most promising alternatives to fossil fuels, and is commonly burned in circulating fluidized bed (CFB) due to the broad fuel flexibility like shape, moisture content, heating value, and other properties. However, at relatively high temperatures, reactions between the silica sand used as a bed material and biomass ash can lead to ash agglomeration within the fluidized bed. While several indices have been developed to evaluate and predict the agglomeration behaviors of coal, these are not necessarily applicable to the actual biomass fuels. This study proposed a method for evaluating the agglomeration tendency of biomass ash based on its reaction with silica sand in a small-scale packed bed setup. To theoretically explain the fundamental mechanisms of agglomeration, chemical equilibrium calculations were also performed. The results show that, for five types of biomass ash, the observed agglomeration behavior closely correlates with the calculated fraction of molten slag. Notably, when the molten slag fraction exceeds 40%, the sinter ratio increases significantly, suggesting that this threshold could serve as a useful screening criterion for evaluating the potential for sintering and agglomeration.