Combustion of three Chinese coals, mixed with limestone physically, was carried out in drop tube furnace. The drop tube furnace consisted of two parts, the top side has a length of about 1.0 m and kept at 1573 K in all the runs, while the bottom-side has a length of 0.5 m and kept at 1173 K SO2 removal efficiency of about 80 and 73% were obtained in the combustion of Yanzhou with high and low sulfur, respectively. In contrast, for Datong coal, the De-S efficiency was only about 50% at the molar Ca/S ratio of 2.0; increasing Ca/S ratio to 3.0 had little effect on De-S efficiency. The combustion ashes were analyzed by several techniques including XRD, SEM-EDX and CCSEM (computer-controlled SEM). A novel calcium-based phase definition, based on CCSEM data was developed to investigate the modes of occurrence of added limestone in the ashes. Additionally, the mixture of limestone with kaolinite was injected into the furnace to study their transformation behavior under simulated coal combustion conditions. The governing mechanisms for limestone capturing sulfur and its reaction with the inherited minerals were correspondingly revealed. It was found that under the given coal combustion conditions, the calcium distribution in the ash varied with coal type and residence time, Briefly, more calcium was used for desulfurization or fixed into mineral; as time progressed, the inherited aluminosilicate, small sized excluded particles in the coal matrix, facilitated its reaction with limestone; it also reacted quickly compared to sulfation of limestone in coal combustion. This in turn hampered the efficient utilization of limestone in coal combustion. (C) 2002 Elsevier Science Ltd. All rights reserved.