A Study on the Effect of Additive Size and Type on Agglomeration and Defluidisation in a Small Fluid BedCombustor

摘要

Circulating fluid bed combustors (CFBC) offer a number of advantages forburning low-rank coals. Particular advantages include their ability tomaintain combustion temperatures (800-900°C) below ash fusion point,control of SO_2 emission by using in-bed sorbents, and control of NOx byusing lower combustion temperatures and air staging. CFBC’s, however,also have disadvantages. Depending on the operating conditions and ashproperties, bed agglomeration takes place in the combustor due to theinteraction of mineral matter in coal and bed material (typically silica sand).Potentially this may lead to defluidisation of the bed. It was established ina previous study that additives such as clay-based minerals could controlagglomeration and de fluidisation during combustion of a high-sulfur coal.Comprehensive analysis of the data suggested that the agglomeratingcompounds were condensing on the fine additives, which were elutriatedfrom the bed.The current work has tested this hypothesis in a laboratory scale fluid bed.Two South Australian coals with high alkali, chlorine, and sulfur contentswere burnt at temperatures of 800°C and 850°C with and without additivesin a silica sand bed. The additives included two grades of fine silica sand,calcined alumina and flyash. From physical observation during the tests,and chemical analysis and electron microscopy after the tests, it wasfound that particle size, rather than type of additives, is more important incontrolling agglomeration and defluidization. Any additive, which is of finesize, and itself is not a low-melting point compound, can potentially controlagglomeration in a fluid bed combustor. Fine additives provide largesurface areas and capture the fine ash or agglomerate formingconstituents, which are elutriated from the bed, thereby controllingagglomeration and preventing defluidization.