The current research work focused on the effect of the heterogeneous distribution of elements on the ash melting behavior of Zhundong coal at the sub-particle scale by using a high temperature stage microscope (HTSM) and scanning electron microscope (SEM) equipped with energy dispersive spectrometry (EDS). Experimental results showed that both molten and unmolten areas existed in the residual ash sample after combustion below 1200 degrees C. Below this temperature, the partial ash melting behavior was significantly found during the ash melting process. Comparing the element composition of local molten and unmolten areas, Na, Si and Mg played predominant roles in the partial melting behavior. The enrichment of Na and Si promoted the formation of low temperature eutectic mixture and further molten areas, while the unmolten areas were rich in Mg. Combined with the micro-Raman analysis, minerals like spinel and forsterite that containing Mg were the main refractory components in the unmolten areas. Thermodynamic calculation results from FactSage software showed that the mass fraction of the liquid phase started to form rapidly at about 1100 degrees C in molten areas mainly with the fast melting of olivine and nepheline. However, the unmolten areas started to melt swiftly at about 1200 degrees C mainly with the fast melting of merwinite. Spinel was the main refractory component in both molten and unmolten areas. (C) 2020 The Combustion Institute. Published by Elsevier Inc. All rights reserved.