The purpose of the new formulas, Cml, CmlK, and CmlY, which express the slowest char combustion rate, is to show the controlling mechanism of single coal burning. Oxygen diffusion through the boundary layer (as a result of releasing volatile matter from coal) to the char surface is the slowest step rate and can also represent as the rate determining. This step has not yet been taken into account in the literature and may effect incomparable decisions between numerical and experimental results of coal combustion studies. In the 1920s, Wilhelm Nusselt found the coal combustion equation for a single coal, which is based on initial coal diameter, and its burning time, or Nusselt square law (NSL). Also, the burning constant in NSL expressed oxygen partial pressure and the ambient temperature level. Nevertheless, recent studies according to char combustion have explained the effect of coal density on char combustion. Consequently, to help understand the slowest rate of char combustion, NSL as well as ordinary char combustion equations can be used together to establish the rate-determining factor. For this purpose, in this study, the slowest step of the char reaction rate is given as “Cml” of stable position for single coal particle, “CmlK” and “CmlY” for a coal particle in a motion.