The flow behavior of high-temperature ash (HTA) and low-temperature ash (LTA) from room temperature to 1350 degrees C was studied. The change of volume in the heating process was investigated by the heating stage microscope. The mineral matter composition and microstructure characteristics of HTA and LTA samples during the heating process were observed by X-ray diffraction and scanning electron microscope. The HTA and LTA samples shrink at the first heating stage, and then melt. With the increasing temperature, the minerals in HTA and LTA samples change and the flowability improves. The HTA and LTA samples show significantly spreading property. However, the shrinkage of HTA sample is different from that of LTA sample. The volume change of HTA sample is not obvious when the temperature is lower than 800 degrees C, while the volume of LTA sample presents the continuous decrease. At the temperature range of 800-1000 degrees C, the main minerals formed in LTA and HTA samples are augite and albite, respectively. Those mineral matter variations result in the obvious volume change of HTA and LTA samples. In addition, the microstructure of the samples shows that the shapes of particles in LTA are irregular and the surface of particles is coarse at lower temperature. With the increasing temperature, the attachments on the particle surface agglomerate, melt and accumulate. The variation of particles validates the change of flow behavior in the heating process. (C) 2015 Elsevier B.V. All rights reserved.