Bed agglomeration behavior in biomass firing FBC conditions

Examensarbete för masterexamen
Materials chemistry (MPMCN), MSc
Tropp, Frida
When fossil fuels are phased out and replaced with biomass in energy conversion in fluidized beds, ash-related problems like fouling, sintering, and agglomeration are likely to increase. Agglomeration in a fluidized bed can cause defluidization and result in high operating costs. Biomass with high alkali content like agriculture residue and waste is especially problematic. Since the composition of the fuel is hard to change, a possible solution to the problems could be using alternative bed materials that counteract the agglomeration tendency of the biomass fuel. Therefore, this study focuses on agglomeration phenomena in fluidized beds using alternative bed materials in an attempt to avoid defluidization of the bed. In this study, two byproducts from the steel industry, blast furnace slag, and oxide scales, were tested as bed materials in a laboratory-scale bubbling fluidized bed reactor. Both fuel and salt experiments were performed at 850°C and 900°C. Sunflower seed shell pellets were used as fuel and were fed to the reactor until defluidization occurred or until 700 g had been fed. The salt K2CO3, a synthetic ash component, was added to the reactor in batches of 0.5 g/10 min until defluidization occurred or until a total of 10 g had been fed. The results from the experiments with the two bed materials were also compared to previous studies performed in the same reactor using silica sand as bed material. After each experiment, the bed was examined and analyzed with scanning electron microscope coupled with an energy dispersive X-ray analyzer (SEM-EDX). In the experiments with blast furnace slag, bed agglomerates were found to have been formed around sand particles and potassium silicate glued the particles together. In the oxide scale bed, potassium gathered around the particles, and only very small amounts of potassium silicate could be found after combustion with sunflower seed shell pellets. Both blast furnace slag and oxide scales proved to be able to withstand higher weight percentages of potassium in the bed before defluidization occurred compared to silica sand. This indicates that they could be more suitable bed materials than silica sand for fuels with a high potassium content such as sunflower seed shell pellets.
bubbling fluidized bed combustion , silica sand , blast furnace slag , oxide scale , sunflower seed shell pellets , potassium carbonate
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