Kaolin addition to a 35 MW circulating fluidized-bed boiler, fired with forestry residues, was performed to study the effect on bottom and fly ash. Ash samples from a period of kaolin dosage were compared with ash samples without kaolin. The samples were analyzed for elemental composition, water solubility, and mineralogical composition. The cross-section of bed particles was examined for elements. The agglomeration temperature of the material from the particle seal was determined. The kaolin, in the form of a fine powder, was supplied to the particle seal. It was found to be carried by the flue gases to the electrostatic filter rather than staying in the bed. Despite this, alkali is removed from the furnace by the kaolin, and thereby, the agglomeration temperature of the bed material increases. The alkali ends up in the fly ash, in which the content of alkali increases and the solubility of alkali decreases, suggesting that alkali aluminum silicates have been formed. These compounds have high melting poi...