Digital image technique was applied to monitor the slagging characteristics of easy to slagging (ES) coal in a pilot scale furnace. Mineral of the ash deposit was analyzed by X-ray diffraction (XRD), chemical composition and microstructure of the ash deposit were analyzed by scanning electron microscopy (SEM) equipped with energy dispersive X-ray spectrometry (EDX), respectively. The ash deposition growth rates detected by the digital image technique in the 4 stages were 0.036, 0.169, 0.039, and 0 mm/min, respectively. The stable thickness of the deposit is 8.2-8.3 mm. The ratio of heat flux reduction to thickness increment were 441.38, 39.45, 23.44, and 0 kw/(m(2) mm) for stages 1, 2, 3, and 4, respectively, which indicated that the initial layer of the deposit had the lowest thermal conductivity. The analysis results of XRD showed that the initial layer (stage 1) was abundant in anorthite and quartz and scarce in mullite and soda-melilite. The components of the layer in stage 2 were similar to those of stage 1 except that no soda-melilite was included in stage 2. Nevertheless, the slag layer corresponding to stage 3 contained only quartz and sodium anorthite. Moreover, the content of mineral phases in the deposit decreased along the deposit growth direction. (C) 2013 Elsevier Ltd. All rights reserved.