In this study, an online digital image technique was used to investigate the process of spontaneous shedding of ash deposits when burning Da Tong (DA) coal mixed with 20 wt % rice hull in a pilot-scale furnace. The morphology of shedding deposit, the variation in deposit thickness, and the heat flux during the spontaneous deposit shedding process were all obtained by combining the digital image technique with an oil-cooled probe. Ash samples were analyzed by scanning electron microscopy linked with energy-dispersive X-ray analysis (SEM-EDX) and X-ray diffraction (XRD) to obtain the microstructure, semiquantitative chemical composition, and mineralogy of the ash deposits. Commercial thermochemical software (FactSage) was used to calculate the thermal equilibrium and obtain the molten slag fraction and viscosity of the deposit. The process of ash deposition can be divided into four stages according to different growth rates. The shedding deposit from the probe at high temperature (probe 1) belonged to ductile deposits, while the shedding deposits from the probe at low temperature (probe 2) were brittle deposits. The duration of deposit shedding from probe 1 was about 4 mm, while it lasted a few seconds for probe 2. For probe 2, the time required to build up a new deposit and shed it was 45 min. The stable heat flux through probe 2 was 180 kW/m(2), and the decreasing heat flux of the initial layer was about 100 kW/m2. It was important to remove the initial layer when conducting artificial shedding. Different sootblowing media were used for removal of ductile deposits and brittle deposits. The SEM-EDX results showed that the shedding deposit originated in the sintered layer, which was confirmed by the XRD analysis results. The main shedding mechanism at high temperature was falling of molten ash deposits. Erosion and thermal shock were the main reasons for deposit shedding at a low temperature. The viscosity of the deposit was a key factor in the deposit shedding process.