Ash agglomeration and deposition during combustion of poultry litter in a bubbling fluidized-bed combustor

In this study, we have characterized the ash resulting from fluidized bed combustion of poultry litter as being dominated by a coarse fraction of crystalline ash composed of alkali-Ca-phosphates and a fine fraction of particulate K₂SO₄ and KCl. Bed agglomeration was found to be coating-induced with two distinct layers present. The inner layer (0.05-0.09 mm thick) was formed due to the reaction of gaseous potassium with the sand (SiO₂) surface forming K-silicates with low melting points. Further chemical reaction on the surface of the bed material strengthened the coating forming a molten glassy phase. The outer layer was composed of loosely bound, fine particulate ash originating from the char. Thermodynamic equilibrium calculations showed slag formation in the combustion zone is highly temperature-dependent, with slag formation predicted to increase from 1.8 kg at 600 C to 7.35 kg at 1000 C per hour of operation (5.21 kg of ash). Of this slag phase, SiO₂ and K₂O were the dominant phases, accounting for almost 95%, highlighting the role of K-silicates in initiating bed agglomeration. The remaining 5% was predicted to consist mainly of Al ₂O₃, K₂SO₄, and Na₂O. Deposition downstream in the low-temperature regions was found to occur mostly through the vaporization-condensation mechanism, with equilibrium decreasing significantly with decreasing temperatures. The dominant alkali chloride-containing gas predicted to form in the combustion zone was KCl, which corresponds with the high KCl content in the fine baghouse ash.