Effect of bed materials mixing on the observed bed sintering
Paper in proceedings, 2014
The most widely used bed material in the existing fluidized bed systems is silica sand. Upon burning fuels with high alkali content, such as biomass, in a bed of silica sand, alkali silicates are formed as molten glassy phase that easily stick and form agglomerates, defluidization of the reactor and unplanned operational stops. Several materials have been tested as alternative bed materials, often containing aluminate minerals. Due to the relatively high cost of such bed materials the most dominant practice is still the use of silica sand even with high alkali fuels and the bed sintering is controlled by continuous replacement of the bed material. The present study investigates the possibility of mixing bed materials and thereby optimizing the existing process by combining the low cost of silica sand with the sintering inhibiting properties of bauxite. Silica sand and bauxite were mixed in different ratios in a series of tests and were exposed to alkali compounds in a 12 MW boiler as well as in simplified experiments in a lab-scale furnace. The performance of the mixture of bed materials was then evaluated based on the tendency to chemically interact with the alkali compounds, the formation of agglomerates and the effect on the overall performance of the reactor. Thermodynamic predictions of the expected compounds at the given conditions were also used to compare with the experimentally obtained results. It was observed that mixing bed materials led to change in the observed mechanism of agglomeration and is beneficial for the fluidization. Furthermore, when calcium was present the amount of the formed alkali silicates and thereby the amount of agglomerates was decreased. It could be therewith shown that a significant improvement can be reached with existing bed materials.