The role of kaolin in prevention of bed agglomeration during fluidized bedcombustion of biomass fuels

Agglomeration of bed material and fuel ash may cause problems during fluidi zed bed combustion of biomass fuels. Previous results have shown that a "st icky" coating, which covered the original bed material and consisted of Ca- K-silicates, was directly responsible for the bed agglomeration during biom ass combustion. The melting behavior (stickiness) of these coatings was ver y sensitive to the potassium content. Frier studies have also indicated tha t bed agglomeration could possibly be prevented by introducing low-cost add itives such as kaolinite. The objectives of the present work were, therefor e, to illustrate the effect of kaolin addition on the actual agglomeration temperature of two troublesome biomass fuels, and to contribute to the unde rstanding of the role of kaolin in prevention of bed agglomeration. By cont rolled agglomeration experiments in a 5 kW bench scale fluidized bed reacto r, the critical temperatures for agglomeration In a normal quartz bed when firing wheat straw or bark were determined to be 739 and 988 degrees C, res pectively. By adding kaolin, 10% w/w of the total amount of the bed, the in itial bed agglomeration temperatures increased to 886 and 1000 degrees C, r espectively. Samples of bed materials, collected throughout the experimenta l runs, as well as final agglomerates were analyzed using SEM/EDS and X-ray diffraction. These results showed that kaolin was transformed to meta-kaol inite particles, which adsorbed potassium species. The increased agglomerat ion temperature was explained by the decreased fraction of melt in the bed particle coatings, i.e., coatings were somewhat depleted in the potassium c ontent by the corresponding potassium-enrichment in the kaolin-derived aggr egates.