Silica and aluminous soil were used as bed materials for the combustion of cotton stalk in a Circulating Fluidized Bed (CFB) combustion pilot plant (0.2 MW). After a long period of operation, the silica bed material was found to have sintered and the aluminous soil bed material was found to have resisted sintering. The particles of both types of bed materials were sampled. Two instrumental approaches (XRF and SEM/EDS) were employed in order to identify structural reasons for the observed differences in sintering behavior between the two bed materials when they were used in the same experimental conditions. From these experiments it was proven that the aluminous soil particles eliminate the bed agglomeration. Although the aluminous soil particles were enriched with alkali metals and alkaline-earth metals after burning for 38 h, they still did not form any agglomerates. Therefore, these particles should provide a good bed material to substitute for the traditional silica bed materials in the CFB combustion of biomass with alkali ashes.