This paper presents the results of the comparative research of combustion specifics of coal-water fuel produced from low-ash and high-ash Ukrainian flame coal. The analysis shows that the effect of the ash content in the coal-water fuel on the duration of the burning of a fuel drop depends on the drop size. The full combustion time of CWF drop based on the low-ash coal can be both less and longer than that of high-ash coal under the identical conditions for different equivalent diameters of the fuel drop. This specific is explained with the domination of different physical factors during the fuel combustion process. The results of this research extend significantly our knowledge of coal-water fuel, allow understanding some issues of its combustion and are important for the design of the specialized energy facility which is used coal-water fuel as an energy source. •Ash content in the coal-water fuel has an ambiguous effect on fuel combustion.•Burning time is the same for different ash content for a critical drop size.•Critical drop size depends on medium temperature of fire box.