Transient Thermal Behavior of Ash During Fluidized Bed Combustion of Poultry Litter

Abstract

When manure is introduced in a hot fluidized bed for gasification or combustion, its inorganic compounds will undergo chemical transformations upon heating. The phosphorus containing salts, which are mostly hydrogen and dihydrogen phosphates or alkali and alkaline earth metals, melt at low temperatures (200–300 °C), before they decompose. In their liquid state, these compounds may drive the formation of ash coatings on bed particles of silica sand, and irreversibly agglomerate multiple particles if they are locally present in high amounts. The recent first time observation of the latter phenomenon led to a new interpretation of an earlier concept, melt induced agglomeration. Decomposition of the aforementioned (di)hydrogen phosphates and reaction of the fresh coatings with silica bed material drives the fluidized bed towards chemical equilibrium upon the ash’s increasing temperature, in which K2Si4O9 is formed aside from Ca3(PO4)2 and CaSiO3. In this situation of (near) chemical equilibrium, a melt is formed if the amount of K2Si4O9 is high and the amount of CaSiO3 is low. Low CaSiO3 may result from a low calcium concentration in the fuel, or by a high phosphorus concentration, since Ca3(PO4)2 is the more stable form. The particle’s silicate melt is concentrated at the interior of the coating, due to the abundance of silica at this location, and any Ca3(PO4)2 makes up the exterior of the coating. If enough silicate melt can find a way through the solid exterior of the coating, entrained particles may deposit onto an inclined refractory wall above the fluidized bed. We support particle deposition as the initiating deposition step, as opposed to gaseous condensation, primarily because of the aluminum and potassium silicate chemistry involved. After deposition, SEM-EDX analysis of a deposit’s cross section revealed a chemical anchoring by chemical reaction between K2Si4O9 and the alumina rich refractory material, creating a strong solid bond of for instance potassium feldspar, which has a high melting point. Deposits can therefore grow larger before they break loose and cause bed disturbances, thus damaging the refractory wall. The comprehensive theory on transient thermal ash transformations, presented in this paper, will allow to adept the design of future thermal energy applications for manure by selecting appropriate additives and refractory bed and/or wall materials.