Combustion of two Chinese coals, without and with the impregnation of calcium acetate, was carried out in a drop tube furnace. Coal pyrolysis was first conducted to study the morphology of the resultant chars, the mode of occurrence of calcium, sulfur and their association within char were studied as well. Secondly, combustion of chars was carried out to reveal the emission of sulfur, transformation of char structure and that of calcium- and sulfur-based compounds in this process. The combustibility of two coals was investigated. Yanzhou high sulfur (YZHS) coal combusted quickly and a portion of the emitted sulfur were captured by the inherent calcium oxide during its combustion. Datong coal (DT) combusted relatively slowly and its sulfur emission was continued until all the carbon burnt out. A thin walled porous structure was formed for the pyrolyzing YZHS char while a relatively thick wall was formed for the pyrolyzing DT char. SO2 emission during combustion of impregnated coals was decreased considerably compared to that in the cases of raw coals. More than half the emitted sulfur was captured by added calcium in coal pyrolysis. Sulfur removal in the following char compounds varied with both reaction temperature and coal type. Almost all the entitled sulfur was captured at 1000 degreesC; increasing temperature to 1200 degreesC, however, decreased its removal efficiency. During coal pyrolysis, the added calcium, which penetrated into coal due to the impregnation, moved to the surface of porous char, where it met the emitted sulfur, capturing it quickly. Little calcium reacted with the inherent aluminosilicate in this step. In the beginning of char combustion, the reaction of oxygen with volatile and char led to the increase of char temperature, as a result, the sulfation of calcium was inhibited to certain extent, whereas the formation of calcium aluminosilicate was facilitated. With char combustion progressed, a porous calcium network was formed, which reacted with SO2 readily, and hence, the sulfur removal efficiency was increased eventually. (C) 2004 Elsevier Ltd. All rights reserved.