We studied the fusibility and flow properties of laboratory ash and Shell gasifier slag from the same coal sample (Chinese Huainan coal). The physical properties of ash and slag were analyzed with X-ray fluorescence, X-ray diffraction, and scanning electron microscopy. The fusion temperature and the experiment viscosity were measured for the ash and slag with the addition of fluxing Cato. Ash and slag have properties that were approximated by the SiO(2)-Al(2)O(3)-'FeC'-CaO system. The computer software package FactSage was used to predict the proportion of solid phase and the mineral phase formed as a function of the composition and the temperature of the SiO(2)-Al(2)O(3)-'FeO'-CaO system. The results show that the fusion temperatures and the temperature of critical viscosity (T(cv)) of ash are always higher than that of slag. Also, the viscosity of ash is always higher than that of slag at the slag tapping temperature range of 1400-1500 degrees C, and the hysteresis between the heating and cooling cycles for ash is more obvious than that of slag because of different physical properties. The fusion temperature and T, of ash and slag decrease with increasing CaO content, whereas those values increase rapidly with CaO content higher than 35%. Also, the sensitivity of the viscosity of the ash and slag with temperature decreases with increasing CaO content because the sensitivity of the phase equilibria of in the SiO(2)-Al(2)O(3)-'FeO'-CaO system to temperature excursions decreases with increasing CaO content. (C) 2008 Elsevier Ltd. All rights reserved.