Circulating fluid bed combustors (CFBC) are ideally suited for burning low-rank coals.Particular advantages include their ability to maintain combustion temperatures (800-900°C) below ash fusion point, control of SO_2 emission by using in-bed sorbents, andcontrol of NOx by using lower combustion temperatures and air staging. CFBC’s are,however, not without disadvantages. Depending on the operating conditions and ashproperties, bed agglomeration takes place in the combustor due to the interaction ofmineral matter in coal and bed material (typically silica sand). Potentially this may leadto defluidisation of the bed. In a previous study, it was established that additives such asclay-based minerals could control the agglomeration and defluidisation duringcombustion of a high-sulfur coal.An experimental study was undertaken to assess the effect of size of additive on bedagglomeration and defluidization. Additionally, it was also decided to examine the effectof non-clay based minerals. Two South Australian coals, Lohiel and Bowmans, wereburned in a laboratory scale fluid bed at temperatures of 800 and 850°C with and withoutadditives. Silica sand was used as bed material, and a suite of additives – silica sand oftwo different sizes, calcined alumina, and flyash were used during the tests. Fromphysical observation during the tests, and chemical analysis and electron microscopyafter the tests, it was found that size, rather than type of the additives, is more importantin controlling agglomeration and defluidization. Any additive, which is of fine size, anditself is not a low-melting point compound, can control agglomeration when used as anadditive in a fluid bed combustor. Fine additives provide large surface areas and capturethe fine ash or agglomerate forming constituents, eventually elutriating away from thebed, thereby preventing defluidization. It was also observed that defluidisation is notdependent on the thickness of coating on the bed particles, and sulfate compounds ofsodium and magnesium, and to a lesser extent sodium chloride are responsible foragglomeration and defluidization for the coals tested.
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