The mixture of coal and lime was ultrasonically hydrated to impregnate calcium onto coal. The calcium-loaded coal was combusted in a fixed bed reactor to examine the desulfurization effectiveness of calcium. The results showed that compared to the mechanical stirring, the ultrasonic treatment significantly improved the SO2 removal, particularly at the Ca/S atomic ratios below 1.5. This is rationalized by the high dispersion of calcium on coal particles caused by the physical and chemical effects of ultrasonic irradiation. (C) 2002 Elsevier Science Ltd. All rights reserved.