Agglomeration mechanism in biomass fluidized bed combustion – Reaction between potassium carbonate and silica sand

Agglomeration is one of the operational problems in fluidized bed combustion of biomass, which is caused by interaction between bed materials (e.g. silica sand) and the biomass ash with a high content of potassium species. However, the contribution of different potassium species to agglomeration is not fully understood yet. In the present work, the reaction between K₂CO₃ and silica sand has been studied extensively by thermogravimetric analysis. The reacted samples were analyzed by SEM-EDX to reveal the reaction mechanism. The results indicated that the reaction occurs in a solid-solid phase already at temperatures around 700°C. The reaction rate increases with increasing temperature, but decreases with an increase of CO₂ partial pressure. Using smaller particle size and well mixed solid reactants results in an increased reaction rate. It is observed that the reaction initiates in the contact area between K₂CO₃ and silica sand, forming a thin product layer. The layer acted as a reactive media further reacting with K₂CO₃ and silica sand. The results provide a basis for understanding of potassium induced agglomeration process in fluidized bed biomass combustion.