Effect of Calcium Addition on Fluidized Bed Agglomeration Caused by Potassium Phosphates

Alkali phosphates formed during combustion of agro-biomass have been linked to bed agglomeration in fluidized beds. It has been reported that KH₂PO₄ can cause agglomeration by melting and gluing quartz bed particles together in a laboratory-scale fluidized bed, whereas K₃PO₄ was reported to cause agglomeration via a reaction with the quartz bed particles, forming a potassium silicate melt with an approximate composition of K₂O·4SiO₂. It has also been reported that limestone (CaCO₃) may counteract bed agglomeration. To study this effect, CaCO₃ was mixed with KH₂PO₄ and K₃PO₄, and each mixture was fed into an electrically heated bubbling fluidized bed reactor. Quartz sand was used as bed material. The results showed a significant decrease in agglomeration tendency for the mix with KH₂PO₄ but an increase in agglomeration for the mix with K₃PO₄. In both cases, high-melting K-Ca phosphates were formed, yet in the case with K₃PO₄, the formation led to an increase in the amount of potassium available for further reactions. The “freed” potassium could then react further with either the bed material, forming molten K silicate, or CaCO₃, forming molten K₂Ca(CO₃)₂. Both compounds glued the bed particles together, while the latter also reacted further with SiO₂, forming xK₂O·yCaO·zSiO₂-containing agglomerates. In addition, the formation of an intermediary and previously unidentified reactive K-Ca phosphate was suggested.