A wall filming mechanism was developed during combustion process in small furnace. The deposited film on the wall was found to be in good agreement with the available data. Slightly higher fraction of film was deposited in oxy-firing due to lower char oxidation rate. The deposited film thicknesses on the furnace wall were found in the range of 0–1.0 mm. The average molten film flows slowly due to higher viscous and surface tension properties.