The effects of the pressure and temperature on the fusibility of coal ash during combustion and gasification were investigated. Experimentation was conducted using a high pressure thermogravimetric analyzer (HPTGA) apparatus. In order to observe the conversion of minerals with changing temperature and pressure in different atmospheres, the resulting ash samples were analyzed using an X-ray diffractometry (XRD) analyzer. In addition, the quantitative XRD analyses of ash samples and a field emission scanning electron microscope (FSEM), together with X-ray energy dispersive spectroscopy (EDS) were employed to verify the detailed mechanisms of ash fusion. The results indicated that temperature and type of atmosphere had a dominant effect on the ash fusion characteristics while the effect of pressure was somewhat more complicated, depending on the temperature and atmosphere being experienced by the ash. The high-temperature minerals such as mullite were formed with increasing temperature under both combustion and gasification atmospheres. However, in gasification atmosphere, there were more fluxing minerals and feldspar minerals present, such as muscovite, anhydrite and K-feldspar, decreasing the fusion temperature. The effect of pressure on the ash fusibility showed different behavior at different temperatures. At 900 degrees C, the decompositions of low-temperature minerals, such as muscovite, anhydrite and oldhamite, were suppressed with increasing pressure, resulting in a decrease in the fusion temperatures. On the other hand, at 1000 degrees C, the low-temperature minerals transformed into high-temperature minerals such as mullite and sanidine with rising pressure. However, the presence of fluxing minerals and the melting of iron-containing minerals resulted in the lowering of the fusion temperatures. Crown Copyright (C) 2012 Published by Elsevier Ltd. All rights reserved.