Among various forms of coal ash and bed material purged from the bottom of pressurized fluidized bed combustion boiler, a kind of the fused agglomerate ash grain was analyzed by SEM, EDAX, and XRD. The agglomerate shell was found to consist of black matrix and white particles. The inner and outer surfaces of the shell appeared to consist of different compositions. Inner and outer surfaces showed ratios of Al and Si to Ca which were 3:1 and 1:1, respectively. The white particle was found to be basically CaO, carrying some Al and Si (Al + Si/Ca=1:3). The black matrix consisted of Al2O3/SiO2/CaO glassy mineral with melting point around 1350degreesC. CaCO3 is calcined to be CaO in the bottom of the combustor where CO2 partial pressure is low, whereas recarboxylation takes places in the fluidized bed where CO2 partial pressure is high, The fine particles of Al2O3, SiO2 and CaO less than 1 mum gather to form sphere, and then agglomerate glass, which lowers melting point of the mixed oxides. Excess CaO stay as white particles in and on the grain. Combustion of coal grains in the agglomerate releases heat and CO2, of which evolution through the melted agglomerates swells the glass to form the spherical shell. Disturbance of fluidization by such agglomerated ash in the bottom inhibits the heat release to raise the temperature beyond its melting point. Such melting accelerates further the formation of agglomerates, which disturb the fluidization through the adhesion in the narrow spaces between tubes to raise the temperature further, finally resulting in the plugging of melt agglomerate to force the shutdown of the operation. (C) 2002 Published by Elsevier Science Ltd.