This paper presents results of an experimental study to assess the effects of type and size of additives on control of agglomeration and defluidization during combustion of high-alkali, high-sulfur lignites. It was established in a previous study that additives such as clay-based minerals could control agglomeration and defluidization during combustion of a high-sulfur coal. Analysis of the data suggested that the agglomerating compounds were condensing on the fine additives, which were elutriated from the bed. The current work has tested this hypothesis in a laboratory-scale fluidized bed. Two South Australian lignites with high alkali, chlorine, and sulfur contents were burnt at temperatures of 800 °C and 850 °C with and without additives in a silica sand bed. The additives included two grades of fine silica sand, calcined alumina, and fly ash. From physical observation during the tests and from chemical analysis and electron microscopy after the tests, it was found that particle size, as well as type of additives, is also important in controlling agglomeration and defluidization. Any additive which is of fine size and itself is not a low-melting-point compound can potentially control defluidization in a fluidized bed combustor. Fine additives provide large surface areas and capture the fine ash or agglomerate-forming constituents, which are elutriated from the bed, thereby controlling agglomeration and preventing defluidization.