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.