Impact of Ash on the Properties of Fluidized Bed Materials

Abstract: The utilization of the fluidized bed technology offers the possibility of thermally converting a large variety of biogenic feedstocks. The characteristics of the bed material which is used in the process play a major role regarding the overall process performance. These characteristics are influenced by interactions between the bed material and the fuel ash. The materials investigated within this work were quartz, ilmenite, olivine, and two different feldspars. This work investigates several different properties of the bed material which can be influenced by interactions with the fuel ash. Depending on the combinations of fuel and bed material, as well as the choice of thermal conversion process, these interactions can be beneficial or detrimental. It was found that quartz and ilmenite have a strong tendency to react with alkali and can thereby mitigate to a certain degree alkali-induced corrosion of metallic components in the reactor. In the case of quartz, this bed material-alkali interaction leads to the formation of alkali-silicates which exhibit a low melting temperature and can therefore cause agglomeration of the bed material. Olivine and feldspar are more resistant towards agglomeration. Interactions with biomass ash led to the formation of an ash layer which has catalytic properties towards tar removal, which is necessary to avoid problems associated with tar condensation during gasification. After longer residence times, the particles acquire an oxygen carrying ability due to the formation of a surface layer containing Fe and Mn. This decreases the calorific value of the product gas and requires bed material replacement. If fuel containing high amounts of phosphorus is converted, this waste stream can further be utilized for nutrient recovery. Ilmenite is used in a process which exploits its oxygen carrying ability. Dilution of ilmenite by fuel ash diminishes this property but the necessary replacement of material can be decreased by magnetically separating active ilmenite from inactive ash.